Take Germany for example: In the neighbouring country Switzerland you can easily earn twice as much and the higher living costs only cancel out a part of the difference. >80% of Germans would have the money to move and would be much better off and still only very few do it (<2%).
Same with all the unemployment in Spain, people could move to e.g. Germany in masses but they don't.
People just don't like change and are not the rational players everyone wants them to be.
Actually there are quite a lot of young people from Spain & e.g. Romanians that leave their countries. Emigration in Romania is quite a big problem (from 23 million to 20 million people). I guess it depends on your definition of "moving in masses".
Did you know? It costs money to move somewhere, and unemployed people have very limited incomes. Stumping up the apartment deposit + bus ticket + float to cover living expenses until first payday might be hundreds of Euros & be completely unreachable.
> People just don't like change
I think people would jump at the opportunity for a well-paid job and are far more rational in their decisions than you give them credit for.