The reason why the tomb has not been robbed already is simply that tomb workers who were making a new tomb covered the entrance to the old tomb with stone chippings and the grave robbers could not find it (this is thought to have happened in real life with Pharaoh Tutankhamen.)
2-Very Well Hidden
There are stories that may or may not be true that those who built the tomb of Genghis Khan were murdered to serve the Mongol Emperor in the afterlife and to ensure that the location of his tomb would be hidden forever. This may or may not be true, but either way, Genghis's tomb has still not been discovered to this day, even after many centuries have gone by, and may lie undisturbed and unrobbed to this very day.
3-Under the Sea
The monarch and his or her grave goods have been placed on a ship which has then been taken far out to sea, where the ship has been scuttled in deep water deliberately, making it extremely hard to reach it to rob it. Many fathoms of water make extremely effective tomb guards, at least in the times before modern scuba diving equipment was invented.
4-Touching the Sky
The tomb complex of the Bemberkar Royal Line is on the top of one of the tallest mountains in the country, with a wall with a small military garrison built around the mountain. If a tomb robber gets over the wall without being caught, he or she must avoid the guards, climb the mountain without being killed by the cold or in an avalanche, rob the tomb and then bring all the tomb treasures down the mountain and over the wall without being caught and killed on sight.
The grave goods are made of a mineral that violently explodes in sunlight-and this is common knowledge. Most people won't want something that blows up on them as soon as the rays of the sun fall upon it and certainly won't want to pay for it.
6-Hot, and not in a good way
The Nawabs of the Lands of Fire are cremated after death and the ashes are buried with vats of molten gold and silver in very hot volcanic areas where the intense heat keeps the gold and silver molten. To rob these tombs, first the robbers must get the vats out without burning themselves or tipping the molten metal over themselves, then they need to either wait for many hours for the molten metal to cool or pour it into a water source, and then the gold and silver is not in a state where it can be directly spent. They either need to forge it into gold and silver coins or at least bars, a complicated process, or sell it for less then half of what it was worth in the black market.
King Aelfric II died of the Scarlet Plague (or of an infectious disease of your choice) and he and his grave goods were carefully lowered into a plague pit so that would be robbers will catch the disease and not live very long to spend their ill gotten gains.
8-Governing from the tomb
When King Chilbert of Agenia died, his son King Edwin missed him very much, to the point where he decided to rule using his father's tomb as a small royal palace. As a result, there is so much security around and so many people coming and going that it is near impossible to rob the tomb.
9-They won't shut up!
In the musical country of Euphonia, grave goods have been enchanted so they sing continuously so noone would want to buy them. Graveyards must be far away from human habitation so that people are not disturbed by the noise, and when you enter one of these graveyards you often hear loud singing of the deceased favourite song in life, which can cause a cacophony. It would be easy to rob the graves, but noone would want to own something that sings loudly all the time, including yourself.
10-The funeral happened yesterday
If the funeral happened recently enough, grave robbers have not had time yet to rob the tomb yet.
The tomb is guarded well for a year and in that time the tomb passages, which have been prepared in advance, grow a nasty kind of fungus, whose spores in an enclosed area are deadly to the lungs, not just of humans, but of all known intelligent races and many animals. Within days of entering the tomb, the robber will be coughing and choking up his or her lungs for a few hours before suffocating to death.
The Satraps of Belshazzar are buried in facemasks and suits of gold-but it is the deadly Vridian gold which is public knowledge. Noone can steal the gold from the tomb without soon dying horribly within hours, and grave robbers leave these royal tombs well alone.
A pair of large dragons has taken over an unrobbed royal tomb as their treasure hoard, and noone can get into the tomb for fear of getting eaten as normally there is at least one dragon guarding the place.
The tomb is full of dangerous and hard to avoid Mechanical Traps which, so far at least, have successfully killed those who dared to try to rob this particular tomb.
15-I can't get near it!
Whoever touches the tomb is teleported a hundred feet away from it-and over a cliff where at low tide, at least, the fall is deadly. To rob this tomb, one must be able to Spellbreak the teleportation spell first.
16-What was I here for?
The tomb has a forgetfulness spell that works on everybody who is not a relative of the deceased, and when they get within six feet of the tomb it makes would be tomb robbers forget that they came to rob the tomb until they are out of sight of said tomb. To rob this tomb successfully, the spell will need to be Spellbroken.
17-The Grave Weapons Fight Back
In life the occupant of this tomb was a warrior king and amongst the tomb treasures are several swords , a spear, and a bow and arrows. If somebody should break into the tomb and try to rob it, the weapons will come to life and attack the intruder, and whilst the bow and the spear can be cut to pieces and *killed* that is not the case for the swords, who will go on attacking the robber or robbers until they either die or retreat from the tomb, due to a spell cast upon the weapons just before the tomb was sealed up.
When the king died, a large number of condemned criminals, plus a small number of people who genuinely wished to die with him to reach heaven, were sacrificed and enchanted so that if the tomb is ever robbed, they will leap to their feet, draw swords that they were provided with after their deaths, and attack. There are too many of them for the average band of grave robbers to deal with, but a skilled bard with the Shoes of the Moon and a Glove of Undead Enchantment could prevent the undead tomb guards from attacking long enough for the tomb to be plundered of it's treasures.
By magic, the body along with any grave goods are shrunk down to a small size that can fit in an urn, avoiding either a lot of land being used up by grave space, or bringing in compulsory cremation which is against the state religion and would annoy a lot of people. To rob this tomb, you must break into the house where it is, steal the miniature treasures and then Spellbreak the spell on the grave goods to restore them to a suitable size to use or to sell on the black market. Often the body needs to be Spellbroken as well to steal the rings from it's decomposing fingers. In a few cases, the body has been entranced to react with that to try and kill the Spellbreaker by strangulation, so watch out.
20-The Undertakers got there first
The undertakers and the local priests were corrupt and stole the grave goods before they ever reached the burial site.
21-Now you see them, now you don't
An invisibility spell was cast on the grave goods to make them go invisible when exposed to light, in the hope that any grave robbers will think that the tomb has been robbed already and leave. The spell will need to be Spellbroken to bring the Funeral Gold and Grave Silver into view.
22-Within the Royal Palace
The royal crypts of the Jade Emperors are right underneath the Palace of the Sun where the Emperor lives and governs the Empire from. So to break into and rob them one must first get past the palace guards, break into the palace, and dodge a multitude of Imperial servants ranging from the Grand Vizier down to the cleaning staff, any of who might sound the alarm.
The royal tombs are protected by a group of Warrior monks belonging to the Mitrall Cult who will risk their own lives if need be to keep the royal treasures that are buried with each dead monarch safe, and they cannot be bribed either. Some of them are always on duty at any one time.
24-The Treasures are fakes
The weapons in the royal graves are broken before internment, and the coins and sceptres are made from lead or wood with just a small amount of gold leaf on it. The royal robes are made of cheap imitation cloth, and the armour is totally rusted. This is publically known, and few would want to risk the fearsome punishments for tomb robbery when the treasures are not really worth very much. Unfortunately the PCs do not know this.
There is only one Undead in this tomb, but it has immense power. Dwarven tombs are often guarded by something like a Dwarven Beard Golem whilst other cultures might have Barrow Wights or something nastier, more powerful, and very hard to defeat.
As soon as the treasure is touched by anyone but the rightful owner or owners, it forms into a mighty Coin Golem, snatches up a jewelled sword, and attacks the would-be thieves. And if the treasure hoard is a large one, then the Golem will be large as well and will take a lot of effort to *kill*, causing it to collapse into a heap of coins again. What is more, unless the Spell is successfully Spellbroken or the coins are placed in small individual bags, every 24 hours the Coin Golem will reform and attack whoever has the treasure at the time.
If the tomb is important enough and recent enough, well paid Royal Guards might be set to guarding it's treasures.
28-A Fey Place
In the case of the Barrows of the First Men digging into one normally reveals just earth and bones. To take the treasures you have to dig into the tomb on the right day when it becomes a link to a world of the Faerie. Whilst the treasures are worth taking, one has to undergo dangerous tests or combats to do so.
The crypts of the Blue Kings were dug by slaves and criminals in radioactive metal, and the treasures and bodies were placed there in the same way, the slaves and criminals dying not long afterwards. Anybody who enters the crypts without lead armour or powerful magical protection will die in the same way.
The tomb gold is in a single, large, heavy Cube mixed with steel or some even harder metal. Dragging it out of the tomb and separating the gold from the base metal will be a lot of hard work.