That is correct, but it takes more energy for a European national to move country because of the barriers of language, customs, laws, tax systems, etc...

As an example, I have worked in Portugal, Spain and now the UK and I don't know if I will ever be entitled to a state pension when I retire even though I payed pension related taxes in all of these countries.

In the US, AFAIK, it's a bit more homogeneous, no?