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.
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.