I think your prior is off...not to mention that if they wanted to, they would have done it long ago.

In my experience, immigrants are a rather unusual group of humans with a unique personality profile - most people wouldn't voluntarily move to a new place where they don't know anyone.

(If you don't think it is that unusual to be willing to move, do keep in mind that America's culture is unusually individualistic.)

Among those who would leave their homeland, America has been a popular destination...but most people wouldn't leave their homeland.