Then why doesn't it limit humans too?
Because something has to be the baseline against which all others are compared. Humans - likely due to the fact we're all pretty familiar with them - are it.
Then what you will actually see is almost no one ever playing that. Which was the whole point.
Exactly. It's my whole point too, only from the reverse direction: playing an against-type class-species combo is going to be unusual, and will present a whole new set of challenges to that character's player.
And, as noted in my post above, it is completely possible to support everyone being Basically Competent at every class, while still preserving the physiological distinctiveness of dwarves vs elves vs whatever. You just have to be willing to allow that there are solutions other than the facile one.
How? Either you make those physical differences mechanically invisible (so why bother with physical differences) or you make the classes not dependent on abilities (so why bother with classes).
Except that that is simply not true--the individual variation within populations of sapient beings would almost surely be greater than the difference between their central tendencies. The bell curves almost surely will overlap. And if they do overlap, then there's no validity to the claim that physiology completely and totally determines what a species is capable of doing. It can have influence, yes--but not be ironclad destiny.
Of course the bell curves will overlap. That said, that a Strength-10 Human is average among its people while a Strength-10 Dwarf is a weakling among Dwarves.
Because there are other solutions besides the facile one. Your dichotomy is false: We can avoid nerfing anything, by instead forcing players to choose between benefits.
So are you saying I should power up everything else to the level of what now stands out as overpowered?
Because when something like Dwarven Wizards stands out as overpowered there's only four solutions:
1. Ban that specific combination
2. Nerf ALL Wizards and-or ALL Dwarves such that the combo is no longer overpowered (and then have to fix loads of knock-on effects elsewhere)
3. Power up ALL other classes and-or ALL other species such that combos involving them are on par with this one
4. Do nothing and just accept the problem as part of the game.
The reason for the ALL in those is that I simply can't justify saying, for example, that only Dwarves who become Wizards don't get anything from high Con; and if I nerf those Dwarves then in the name of setting consistency I have to nerf all Dwarves equally. (thinking on it further I think I'd have the same problem with the idea of 13th-Age Necromancers not getting the same benefits from high Con that everyone else gets)
Of the above, IMO 4 isn't even worth considering; neither is 3 as I'm specifically trying to fight against power creep rather than add to it. 2 does nobody any favours. But you seem to be arguing for 3, and yet how can 3 happen without up-powering the whole game?