I suspect that's because Europe is hugelly varied whilst the United States are, in what's actually almost twice area, much less varied in terms of culture and values (for example, the whole of North American has all of 3 main languages - with English clearly dominant - whilst Europe has over 20 main ones plus another 80 or so minor ones).

Living in Europe it's very likely that you'll actually cross paths with and even know well people from the country of your ancestors (plus from lots of other quite different countries) and lose all illusions that you're culturally the same, whilst in the US one can live in blissfull ignorance thinking eating spaghetti and having an Italian great-grandfather makes them a lot like Italians, never actually having met and gotten to know well an actual modern italian.

It's actually funny: people within a specific cultural environment have a tendency to spot in great detail everybody's slight differences, which for outsiders are pretty closed to unremarkeable, and it's only when you go live elsewhere do you notice all those "great differences" were nothing at all compared to the differences in people between countries, at least in Europe. It's actually funny how for example my keen spotting of regional differences in my home nation of Portugal (which is tiny yet even that one has such things) suddenly became silly when I moved to The Netherlands, by comparisson with the great differences in people between the two countries, and ditto when I moved to England, and then as I lived longer and longer in those countries I started spotting the regional difference in people within those countries (and in the special case of Britain, the differences between people from the various nations also became sharper in my eyes).

I suppose things like an Italian-American subculture come from that keen spotting of what for outsiders are quite small differences and then that mixed with profound ignorance on the subject matter makes many confuse being "an American with a drizzle of Italian" with being part Italian.

Mind you, it's all valid. It's just that for me who have lived in a couple of countries in Europe, been to quite a few more, can speak several european languages and know people who actually grew in various countries in Europe, that kind of identification with the nation of one's ancestors in the US looks quite ill-informed.