In Paul Cooper's Fall of Civilizations podcast episode on the Aztecs, he mentions that horses initially evolved in the Americas, migrated to Asia, and then died out until they were reintroduced by Europeans. Cooper mentions without elaborating that they may have been over-hunted to extinction by early Americans.

I feel unsatisfied at this explanation. Humans tend to be quite good at figuring out all the things you can do with animals - with all the monumental benefits horse riding/breeding would bestow, what factors might have contributed to this never being picked up before it was too late? While I know the answer to this question will ultimately be speculation, I'm curious if anyone has insight into environmental/geological/etc. factors that might have led to this conclusion.