Not my area of expertise, but I think the intuition for why evolution didn't make us all Einstein's comes in two parts:
(1) IQ is very polygenetic - a big IQ gene might be 0.1 IQ, and there aren't a lot even that large, if I recall the large genome wide association studies done on this. As GWAS studies get larger we find more, but they're all super small effects.
(2) Having an extra 0.1 IQ point is easy to lose in the noise of other effects on having offspring.
So even if we assumed IQ was perfectly correlated with more surviving offspring in modern societies, it might take an extremely long time (even longer than a whole planet and a thousand years) to make a detectable difference.
(1) IQ is very polygenetic - a big IQ gene might be 0.1 IQ, and there aren't a lot even that large, if I recall the large genome wide association studies done on this. As GWAS studies get larger we find more, but they're all super small effects.
(2) Having an extra 0.1 IQ point is easy to lose in the noise of other effects on having offspring.
So even if we assumed IQ was perfectly correlated with more surviving offspring in modern societies, it might take an extremely long time (even longer than a whole planet and a thousand years) to make a detectable difference.