Why do you think that middle class Europeans can't become middle class Americans? I know more than one example where that was no problem at all. It's not like a middle class American or European could just move and then not work and just live off of their savings. They move and get a job (ideally the other way around), jobs tend to pay relative to the living cost of where you live.

That's why people compare spending power instead of how much money you earn, because that comparison is meaningless.