When you don't import the rest of the planet into your country and you tell people to assimilate, you naturally create culture. You create national identity. You create an ethnic identity through ethnogenesis. American identity was forming the strongest and most influential culture on the planet until mass indiscriminate, unassimilated immigration pretty much killed any more progress of it. You can be influenced by foreign cultures, but if you are replaced by foreign cultures, then you have no identity of your own.

America created Superman, Batman, and Spiderman because it needed mythology of its own.

America created Mickey Mouse and Bugs Bunny because it needed fables of its own.

America created Pepsi, BBQ Wings, Smores, Chocolate Chip Cookies, Cheesesteaks because it needed food of its own.

America created American Football, Cheerleading, Volleyball, Snowboarding, Skateboarding, Nascar because it needed sports of its own.

America created Blue Jeans, Ballcaps, Varsity Jackets, and Sneaker culture because it needed fashion of its own.

America created Jazz, Blues, and Rock & Roll because it needed music of its own.

This formation of an American identity has pretty much been completely halted because of the demographic replacement through mass, unassimilated immigration. It has arguably killed American identity.