But the USA has massive federal tax revenue and spending which redistributes wealth from rich states to poor states. Federal taxes are much higher than state taxes.
By comparison the EU has high state taxes, but very tiny "confederal" EU level revenue. So much less ability to balance things between rich and poor member states.
This puts poor states like Portugal into a bad position where they suffer from brain drain due to the free movement, but don't get enough monetary benefits from the EU to counteract that.
This turns Portugal into a gateway country for the EU: they have loose immigration rules because they want to attract young workers. But as soon as those workers live in Portugal long enough to get an EU passport, many brain drain away to other (better paying) EU countries, and need to be replaced with even more immigrants.