When the Amerindians got past the glaciers, it was as if they had died and gone to heaven. They’d lived in Beringia for thousands of years, so obviously it was possible to make a living there, but it was a harsh and unforgiving environment. Things were way better south of the ice: more game, game that had never seen humans, never co-evolved with humans, had no hereditary fear of humans. Likely a brave could close to atlatl range of a giant ground sloth without any reaction.
On the other hand… there were a lot of predators around, and they didn’t have any baked-in fear of humans either. Dire wolves, sabertooth tigers, scimitar cats, the American lion, short-faced-bears. Today, big predators in Africa and Eurasia know in their genes that Man is dangerous. In America, they probably thought that he was the weakest and most defenseless of all living things, and acted according. The life of those early Amerinds was thus full of interest.