In Europe, there are 217 distinct recognized ethnic/ regional groups- between geographic, lingual, and religious variations and combinations there of. China, with its huge population and cultural homoginy has something like 60. The US is said to have 29 large groups, and 820 smaller ethnic/ cultural groups. (The numbers may be off some, but I remember them from a credible source). You tend to marry people in the same geographic area (duh), as well as the same religious and identified groups. This tends to create a variation between these people and those around them.
Think about it. can you recognize people for their ethnic or regional grouping by appearance or accent or other action? How many groups can you recognize?
Yet in your average fantasy games, with a continent larger than Europe, you might have one (all humans are one continuous cultural and social mass) or even three (one for each human society). Even if these societies are antagonistic, they seem much the same. The only variations you get are the non humans, who seem much like humans in alien suits. All the species are fairly the same, culturally speaking.
Most people can't seem to handle a realistic level of diversity, especially when they are making the game world. Most fiction does not do it, but then again... it does not need to, to tell its story in a limited geographic space and time. One of the few fantasy books that comes close is Jordan's Wheel of Time. Each region has its own meta-culture, or cultural themes, and each country has its own unique culture. the Northern cultures all have their distinctive knots and garb (from their ancient tribal herritages). The Southerners, far away from the dark lands, have become self absorbed and europeanesk. Mercedes Lacky's Valdemar tales have a great deal of cultural diversity implied, and occasionally seen. So we know it can be done, yet why don't people play it out?
You don't have to be an alien to be an alien. Take someone from 1300 Northern Europe and drop them in Japan or China of the time. A fur covered alien might be better received. Cultural and Lingual differences are enough to make people mutually un-understandable.
You don't have to be another species to be different. Think about the differences between the Nordic Countires, and Greece in terms of appearance of the population. North Africa in comparison to more southern regions. You can notice the differences.
There are game examples:
In Kerren, there is a little genetic diversity in the group. They all came from the same region of Terra. There are some regional appearances and ideals (Westerners, Southers, Pointers), but they are similar cultural and social groups, but there are regional differences in cuisine, accent, and dress (based on weather).
In my fantasy world, I have a group on Sixthland, the Lyrans. They are olive skinned, large almond shaped eyes, long of limb and digit, graceful, strong. There ships traverse seas humans can not cross. They have artifacts that Humans do not have. The Humans think they are a subtype of Elventi (there general superiority, nature affinity, and magikal technology), while the Elventi think they are Humanti (round ears, shorter than Elventi lifespan-but longer than Thirdland and Secondland humans, and they use technology). Many people on the world think they are a different race. They are an ethnic group of Humanti, blessed in terms of geography and history.
Do you see how difference can not be so different but make the people seem of a different species.