They had the last millennia to homogenize and it didn’t happen. The United States really are very much unlike Europe. That’s not to say it won’t happen but I don’t think it’s a foregone conclusion.
Yeah, but it's been what, 60 years since they stopped trying to invade each other? I don't think it's a foregone conclusion either, but I think it's definitely the trend the EU is on.
A lot of the countries in Europe have strong independent governments that were established long before the USA existed. European countries have large complex systems of law that have taken thousands of years to develop and grow. We also have hundreds of millions of people. What was the original population of the USA?
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.
You could have given the same arguments 50 years ago to say Europe would never have a successful political union, continent-wide military alliance, or currency, and today they have all three.
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.