Because intrinsic ability is such a vanishingly small part of the equation that we can't know who could actually be the best until we actually give everyone a fair shot.

There might be the rare generational talent that, starting in their discipline at age 18 with no prior exposure and poor nutrition, education, health, exercise, etc, could outcompete your average loser brought up with every advantage and private lessons from age 6, but in general I wouldn't expect talent to out in those circumstances.

And school's not supposed to be about filtering for rare generational talents, at least not first and foremost. It's supposed to be about getting everyone as far as they can go, and if we separate people into "smart" and "dumb" buckets before they're old enough to ever have actually gotten a chance, some people will be stuck in the "dumb" buckets their whole life that could've been a solid contributer to society if society ever cared enough to invest in them.

Or, another way of looking at it: Everything else is made to put a thumb on the scale. Everything else is designed from the ground up to advantage the advantaged. Public school is supposed to be one of the few institutions that mitigates that, that tries to put a thumb on the other side at least a little, to help level things out. And the people with the advantages hate that, and try their hardest to thwart it, whether through private schools, through pushing public schools to make different "tracks", or whatever.

>Why do we have such an easy time accepting peoples intrinsic athletic ability and such a difficult time accepting people's intrinsic mental ability?

You know why. It leads to something so heretical even alluding to it could cause irreparable harm to your reputation.

This broadly true but economy isn't run on NBA, NHL, MLA, i.e. a few 1000 of 5 standard deviation talent where separation is mostly genetics. Academia need to develop magnitude more passable high end workers, the genetic pool for that is large and system biases towards culture to fill 1,000,000s of 1-2 standard deviation brains. You need to hammer minor leaguers to see if they make it to rookie league or whatever level below AAA that system has demand for. Reasonable system would be to herd everyone through filtering process and throw drop outs into vocational training or soft subjects that should not be elevated on same level of STEM, not because they're less valuable people blah blah, but the pipeline should distinguish and prioritize strategic sectors.

No one is saying there isn't, but it's objectively a stupid massive oversimplification of how complex things like a human brain and human learning really are.

For one, people used to be a lot better, do unless you think people are actively dumber, you argument doesn't hold.

School capabilities also correlates massively with things like access to resources and wealth of parents, and inversely with mental health.

We also have very strong incentives as a society, as an economy and as a democracy to have as many educated people as possible, to work on setting the best conditions possible for people to learn