Mm, over the years, I've known some people who were legitimately kinda dumb. One learned, at the tender age of 27, that flamingos weren't mythical a la dragons. On the other hand, I've known some people who were absolutely brilliant.

You might as well argue everyone can run the same speed for the same amount of time. If you haven't observed this yourself, I would think you've probably just travelled through life in very homogeneous social circles.