It's true that America isn't only New York City, Los Angeles, and San Francisco, but those three areas do make up over 10% of the population, which is nothing to sneeze at.
In fact, if we generously estimate that 10 million people live in high density metro areas that don't rely on cars we are looking at less than 3% of the US population. I think that is something to sneeze at.
I know it's hard for you to believe, but some people like living in high density areas.
SF only has a population of one million.
Basically the US has NYC and parts of Chicago as emblems of urban density.