I've been thinking about this for a while. While the US is such a huge country, it's obviously hard to pinpoint an obvious "American culture", but even more so maybe the identity is so hard to track down because of its prevalence? During the last 100 years American culture's become so prevalent in the west ("globalism") that a lot of countries are very Americanised where popular culture, art and even businesses originate from the US.
I mean, I live in Eastern Europe and I'm currently watching an American TV series on Netflix while I'm writing this comment on my Apple MacBook with a podcast waiting for a listen later and there's a Sam Harris book on my bed. It's nothing like my heritage of what it means to be Hungarian, living in Transylvania. And yet you're sharing a lot of American identity with me.
It's just a thought I've been playing around with, but maybe that's why Americans have such a hard time finding their identity as Americans?