when the world becomes too much of a kitchen sink

[Pelle]

I don't know about that. I've never met a player with that assumption.

Most players ask me about the setting and what kind of races/monsters live there.

There are plenty of players on these boards with that assumption. As for in person games, depends on your group. Mine don't.

It is a lot easier for everyone to include all the races of the core rulebook in the setting, and I try to do that. Though I find it hard to make people accept/realize when they are slightly refluffed or have different cultures than presented in the rules. It would be easier to remove the races completely to avoid peoples preconceived views of them.