I'm not talking about kids. I'm talking about young infants, that for instance, learn to read or do basic mathematic operations on their own. For those kids, the rationalizations you propose don't apply. There is also the ability to grasp concepts quickly, that regardless of training and mental gymnastics, is different from person to person.
Sure, as I said, training can do wonders, and probably the apparent difference will decrease. That's why hard-working students with average intelligence do better than "real smart" students that don't dedicate enough time to their studies.
Also, creativity when approaching new problems and situations can't be trained. And it's undeniable that some people are better than others at that. Adaptability can't be taught. For that matter, neither can abstract thought capacity, musical talent, etc.
Okay. But the opposite is true, as well. Not every old concept is necessarily wrong just because it's old.
It is convenient because it's true. There is no such thing as equality. It's all a fallacy invented by people who were tired of other people using feeble, random arguments to support their whimsy power. It served a purpose back then, and I guess that, to some degree, it does today. But it's still a fallacy nonetheless. Nature isn't fair, as "fairness" is a human notion, and a rather random one at that.