The reason everyone thinks Americans have no culture is because their culture is prevalent everywhere.

I bet it's more to do with how little Americans own their own culture. Copyrights in the USA used to expire after 30 years, after which it became public domain. Or in other words, culture was returned to the people as a whole.

Nowdays the copyrights last beyond a lifetime, and Americans grow up in a world where they almost never experience relevant pop culture outside of being owned or controlled by someone. When you find American content, you don't think of "American culture" you think of "This is owned by Disney" or "This is owned by Paramount" and so on and so forth. You have original authors and content creators, being the gods of the world they created, and everyone else are "fan artists" or "fanfic writers," being implied to be lesser. Those fan artists will be fan artists their entire lives, and their works will never be 'canon' in the eyes of the Owners. If you like Harry Potter but not Rowling, too bad. The public cant reclaim it.

That's not how culture works though. Culture remixes, reinspires, deconstructs, rebuilds, and memes on. That's how everyone did stuff before the advent of recorded media. The good stuff is repeated and boosted. In a way, the Internet culture that emerged in the 90s sought out to rebuild what was lost after the 1890s.