By comparison e.g. Spain and Germany and almost any two countries in the EU you care to name speak different languages, are very different culturally, and have different customs.
Apart from a few of the more 'third world' european countries (like France) if you speak english and work in high tech it's not a big problem to work in Scandanavia/Germany/Netherlands. The main difficulty is they drive on the wrong side of the road - but as consolation the coffee and women are much better.
How is France a third world country in any sense of the word? Because they don't speak English as much?
> The main difficulty is they drive on the wrong side
That's only in the UK, not Scandanavia/Germany/Netherlands.
It was 'irony' - France is notorious for obeying only those EU laws which benefit it. In particular there are lots of unofficial barriers to other EU individuals/companies working in France - even though they are legally allowed to. Is't common for various levels of government to delay required licencing for years.
It's an issue even with international collaborations - the European Space agency is in the Netherlands and CERN is officially HQed in Switzerland - there was a lot of concern over ITER before it was built in France, the French 'promised' to play nicely with people working there.
How difficult would it be to wipe and rewrite the constitution of the USA today? Imagine something 100 times more difficult than that, and that is how difficult it would be to create anything resembling the USA in Europe.
Incidentally, I don't know about "a lot". Many European countries formed as unions of smaller states not that much longer ago than the colonization of America--the UK, Spain, Italy, and Germany all formed this way, Germany after the United States and the UK only a couple decades before. And nearly every country on the continent of Europe has already had its constitution rewritten within the past century.