'American' is a civic identity. The general idea is since modern americans (primarily descendents of western europeans) took the region from the actual, native americans, nobody has a right to expell others from being american, as long as you pledge allegiance to America. Therefore immigrants are less anxious about assimilation. Because the framework is clear. It's also one of the reasons why muslims are much better integrated in the US than in Europe.

In Europe, identities are more based on shared ethnicity, history and shared ancestry. That's why it's difficult to ever be fully accepted as an immigrant, which in turn triggers a self-preservation impulse in them. Which is completely understandable in my opinion.

Even in the US, you have populations like the hawaiians who are located in America but who don't necessary feel american and who are rather proud of their own distinct culture. Which is also understandable. People vary in their connection to their ancestors and their desire to continue said culture.