Europe isn't really a monoculture or a set of monocultures either. Over it's entire history, cultures have mixed and seperated all the time. I don't think the "melting pot of cultures" is any different here than in the US other than the US has less history with it.
It's different because in Europe you have people whose ancestors have lived in the same village for hundreds or thousands of years. They walk the land where their language was invented.
Maybe their ancestors aren't really as pure as they think, but it's still fundamentally different from a country where everyone is an immigrant within the last few generations.
People didn't really stay in the same village all the time, there is plenty of mixing going on in history. Languages have moved as well, the german-french border can write you a long essay about that.
Considering the current political climate in the US, the difference isn't that large...
