[huherto]

There seems to be an implicit assumption that every body wants to emigrate to the US.

I don't that is true. If given the choice a lot of people would choose to stay. Sure, you can have more material things, but you will have to go to a strange place, far from friends and family, where you are just a stranger, that possible don't even speak the language.

People who decide to emigrate is because they have strong reasons. Either your situation is desperate or your are very courageous, or both. Leaving your country is really not easy.

[threedaymonk]

It's a manifestly false assumption, too. We've had free movement in the EU for quite some time, and yet Greece is not empty. Migration waxes and wanes with economic factors, but most of the population stays home, where their family, language, and culture is, regardless.