Being born here in America doesn't make you an American. The Ugly American. Thus, the identity-politics reflects not a truly different culture, but is a metaphorical way of expressing dissatisfaction with the injustice of divisions within our society. A heavily restricted set of roles within the nation is thought of as being a not a part of the overall society, a separate nation. But being one nation does not mean the absence of internal divisions.
Few countries are homogeneous.
America is notable for the wide range of its population, but such a spread is not unique. There is a difference between uniformity and commonality . If we assume for the sake of argument, that there is a such a thing as a core culture, then more homogeneous countries simply have less expressed beyond that core when compared to more diverse ones. Even the separatists often want to simply replicate nearly everything in the society from which they are supposedly breaking off, but with their group more widely distributed within those structures. This sort of overall unity is thrown into high relief by comparison with a situation such as the area which once was Yugoslavia.