The most important reason to me is the out-of-game reason. There is a limit on how different the game world can be from the real world for me to still enjoy it. There's a limit on how much time I want to spend orienting a new player to the game world.
So, e.g., I tend to make the game world calendar very close to the real world one--or at least provide a "mapping" so that I can use real world terms until the players become for familiar with the world.
In most games I play, non-human characters are one of the many bits that is kept the same (or mostly the same) in order to build interest with changing other bits.
Although, I've known at least one gamer for whom playing a non-human character was on of the most important things. He was very unhappy if I ever tried to run a no-demihumans/no-aliens game. As much as I tend to prefer humanocentric games, I think a no-humans game could be fun too.
Playing a game in which the game world is very alien would be a lot of fun. But (1) Gaming is just a hobby for me & (2) A whole group would have to commit to it & find consensus on the world to choose. Even running a
Lord of the Rings game has left at least one person in the group feeling a bit left out because they didn't know the milieu as well as eveyone else.
In game, I don't give much thought to the rationalization. Perhaps humans aren't dominant, it's just that the PCs are human. (I've got some notes on such a world.) Perhaps it's some stock elder-races-dying-out or humans-infinitely-adaptable rationale. Perhaps it's just up to each character who cares to ponder such issues to develop his own theory through observation of the game world.
