>>Can you imagine if US citizens had to get a visa to move from New York to California?
That's categorically different. The US is united under one national identity, so the ties between states are much stronger. Whereas the EU consists of many different countries and cultures, and each one has a long history of nationalism and rivalry with neighbors.
You've been downvoted but as European I can say that the part of your comment about Europe is true and that's why we should be careful about taking the way of nationalism again. With the EU we fixed the major problem (wars), anything else is less important and can be fixed with some good will. Not that having to go through borders and possibly multiple currencies and tariffs every time we move and buy won't be a major pain in the ass, still a less major problem than others.
That's categorically different. The US is united under one national identity, so the ties between states are much stronger. Whereas the EU consists of many different countries and cultures, and each one has a long history of nationalism and rivalry with neighbors.