Why were people living in America?
Curiosity & Threaten makes people living in America.
Curiosity: Homo Sapiens A, "the Bering Strait is shoal, let's go across it and have fun in America."
Homo Sapiens B,C,D,E,F....Zn, "okay, nice idea, go go go, let's do it."
Threaten: not enough food for the Homo Sapiens A group, eg.: they are not strong enough to fight with other group, and forced to not live in Asia any longer, they had to find another place to live in, America is a choice.
Not sure if you mean why are people living in America, as in present tense, or why did early people live in the land mass that has come to be called America? Or possibly you meant, when America was "discovered" in 1492, what made people settle there?
I'm from America, both my parents are from America, on one side my grandparents are from America. On the other side, my grandparents immigrated to America from Turkey.
America was and is a very enticing place to live. There are certain freedoms there, that are not available in other countries. It's relative modernity, means you don't need to come from an old established family to have prestige. One can be anything or anyone, with many possibilities and opportunities. It was a great place to grow up.
Having said that, I now live in the UK and love it.
Migration in prehistoric times was no different than migration today. Tribal fighting, food supply, curiosity all lead people to try a new place.