The US does not have the highest incarceration rate in the world: https://en.m.wikipedia.org/wiki/List_of_countries_by_incarceration_rate
It's 4th, and the difference between the US and the next few dozen isn't that significant.
Also, you are assuming cause and effect. The US is a rich, well developed country with an unusually high black population due to slavery. The much simpler explanation is that the black population are much more criminally inclined. The fact that their average IQ is significantly less is explanation enough: criminality is closely tied to IQ.
