>>719011895
That's the point, horses had arrived to america before humans, and they hunted them to extinction, which meant they didn't have any transport animals for the develpment of any civ in America.
They had to wait for euros form a second chance.
And not only horses,a lot of big fauna got hunted to extinction too.
Africa and Asia were spared, but humans were too good at hunting by the time they arrived in America and Australia.