7 Reasons Why Aliens Have Not Come to Earth Yet

1-FTL speed is impossible.

If Faster then Light speed is impossible, then aliens can't get to Earth because of the enormously long distances between Earth and their own planet. There could be thousands of alien civilisations out there in the universe, all wondering if they are the only one out there and all of them too far away to contact each other.

2-Earth is a Strict Nature Reserve

Like those areas in Yellowstone and other places on Earth where humans are not allowed to protect the flora and fauna, the whole of Earth might be considered a strict nature reserve by the rest of the galaxy. If UFOs are seen, it's the equivalent of someone going off the boardwalk in a geyser basin and said drivers risk getting fined by the Space Rangers and perhaps losing their pilot licenses if they get caught.

3-We have nothing to offer the aliens

To the other advanced races of the galaxy humans are a backward, violent race with nothing to offer them in terms of technology or resources so they can't be bothered to make contact with us.

4-They are here but in secret

AKA Men in Black. The aliens are here but very much in secret, certainly from most people and perhaps from our governments too, staying hidden to avoid causing a worldwide panic.

5-Disease fears

The aliens don't want to spread Covid-19 or some other nasty disease all through the galaxy, and nor are they cruel enough to want to spread potentially deadly disease of their own through Earth.

6-Alien races are all in hiding from each other

All the alien races are hiding from each other in fear of the danger of interstellar war and things like asteroids being steered into their planets or worse. No-one wants to be the first to go public and announce their location in the galaxy.

7-The cost

Perhaps it's just that FTL interstellar travel is extremely expensive and alien governments would rather spend their money on other things, be it social programmes, military forces or other things.