There are many reasons but one of the most important reason is that the PC's are already atypical.
There's no guarantee of that; DCC makes a big example of how all PCs are random villagers, zero-level butchers, bakers, and candlestick makers. In the current Pathfinder game I'm in, all the PCs are in the same class of graduates from an adventurer's college. At the start of the Pathfinder AP
Mummy's Mask, the PCs are one of a number of adventuring parties looking to get into the tombs. In some games, heroes are made, not born.
Edit: Also, that PCs are already in some sense atypical doesn't mean that they should be more atypical. That you're going to be the best wizard in the world doesn't mean you should be the only dwarven wizard in the world.
I have no problem with removing whole races as playable for the PC's or whole technologies from the Gameworld. I also have no problem excluding Dwarven Wizards providing Dwarves are incapable of learning magic. My objection is due to the actual reason you are limiting players from those races when there is no good reason to bound players to play something typical in your world.
For one, that's not my reason, and I've never given it as such. For another, I don't see the distinction; why can you have the only non-elven bladesinger or the only dwarven wizard in the world, but not be the only warforged or have the only laser rifle?
I can't tell you enough about those worlds to answer that. If you know them so well then give me a good reason for how they survived?
I don't know them so well. I could make an argument about how the dwarves survived, but as I said with Krynn, there's already the ludicrous issue of steel pieces. Most D&D economies fall apart on poking at them, but Krynn's literally has steel swords cost less than the precious metal in them. Krynn is in many ways not a well-constructed world. That didn't stop it being a main setting for three editions (and getting its own non-D&D RPG for a while), with enough love for a movie adaptation.
Edit: (Who is going around killing off races that don't have wizards? Pre-3rd edition, humanoids (as opposed to demihumans) didn't generally have wizards. And dwarves are nestled into the mountain homes so tightly that any assault would be very challenging, even with some magic, and humans don't want to live in former dwarven homes. So much better to trade with the dwarves than try and dig them out.
But this is really all besides the point; the point is it is that way in several settings that people have played a lot in, even if you find it unrealistic.)