There are millions of people in American cities, and tons of people move from the less dense suburbs to these cities.

There are probably even more people who would move if they could, but our cities are expensive (because housing is expensive when you don't build it) and so they stick with the "default". Staying put, getting a car, and finding a way to make it work.

You can't conflate "this is what Americans want" and "this is largely the only choice most Americans get".