As yourself and others have pointed out, there are American realists at work in cinema right now. You mention Reichardt, and then there is also Tim Sutton, not to mention early David Gorden Green films, such as George Washington and All the Real Girls. Plus, there's been triple digit so-called mumblecore films released in the past 15 years. (Also, let's not forget Cassavetes and his world-spanning influence.)

That being said, you're right, it's predominately indie cinema.

I think a lot of it may have to due with AUDIENCE expectations. For example, if I want a gritty realist film, I personally turn to Europe. If I want something surreal, maybe free form, I turn to Asia. Now Europe and Asia have plenty of genre films that are of high caliber, but for whatever reason the connection has been made that certain regions produce certain types of films, and I don't think I'm alone in this. I just named plenty of realist filmmakers and movements here in America, but I never think to myself to explore them or dig deeper to discover new ones, because when I think of American cinema, I just think of big glossy franchise films that just don't appeal to me. IE, all that money studios spend in advertising and going for wide releases works.

The one thing I disagree about, though, is the literature aspect. Going back to Melville, and passing through Faulkner, Fante, McCullers, Carver, and more currently O'Nan, there have always been and continue to be warmly received and widely read realist authors.

Which still brings us to the big question: why hasn't realism taken off in cinema? I think the question can be reframed as why has predominately escapist entertainment taken off? And my theory on that is because that's what's in our DNA. Our ancestors came here to avoid whatever was happening where they were originally from. Escapism is literally in our genes.