The US believes explicitly - or likes to pretend it does - that immigration is good, probably for the obvious reasons that it's a good source of cheap labour. Even though in practice communities seem to stick together even more than they do in Europe.
This proves how useful it is to have narratives. Even if they're nonsense, they make people behave in reliable and useful ways.
Still today, in countries around Europe you see that dynamic at work, or at least advocated by a sizeable political minority.
I think in the US you're starting to develop some of the same, with the "English Only" movement, and so on. Can the last politician to go out the revolving door please turn the lights off?
The key difference seems to be that patriotism is still a mainstream value in the US. We're much more suspicious of it in Europe. Aside from the racists, hardly anyone in the UK considers themselves patriotic, except in a negative 'At least we're not European' sense.
I'd guess - based on speculation - that France and Germany are mid-way between the two.
Without patriotism there isn't really anything to be assimilated into.
/ After 9/11, Gerhard Schröder declared "We're all Americans now" and I was like "Huh? I'm having enough trouble with being German, thank you." "If you know your enemies and know yourself, you will not be imperiled in a hundred battles." Sun Tzu
