The US _always_ lacked a cohesive cultural identity, it has always been manifold.
Basically the rise of television and movies post-WWII-ish depicted a single culture but it was just excluding everyone except essentially WASPs. This had nothing to do with reality and was just racism. Before the world wars there was even a considerable amount of greater cultural diversity among European immigrants and descendants, German being spoken very widely across the country and quite a bit more of people retaining the culture of their ancestors.
The US _always_ lacked a cohesive cultural identity, it has always been manifold.
Basically the rise of television and movies post-WWII-ish depicted a single culture but it was just excluding everyone except essentially WASPs. This had nothing to do with reality and was just racism. Before the world wars there was even a considerable amount of greater cultural diversity among European immigrants and descendants, German being spoken very widely across the country and quite a bit more of people retaining the culture of their ancestors.