I feel like none of the replies to my comment read my second sentence.
Yes, there are many cities in the US with enough density to support public transit. Most of them have it.
My point is that public transit is not a large part of American culture and history which significantly depresses demand for it.
Most Americans grew up in families that owned a car or two and used them to get everywhere. When you ask an American how to get groceries, their immediate mental image is getting in a car, taking a long drive to a big store, and filling up the car with a week or two's worth of stuff. Tell them they can't own a car and all of a sudden they don't know how to solve mundane problems like this anymore. There are other perfectly fine solutions, of course, but they don't know those solutions yet. It's not part of their culture.
Also, most Americans live or at least grew up in cities whose urban planning was designed around automobiles and parking lots. They are spread out to make room for parking lots, which in turn makes public transit slower and less economical. People need cars because cities are too spread out for transit because people prefer to drive because they have cars because cities are too spread out...
There is a huge path dependent effect here where most cities aren't laid out for public transit and most people don't prioritize it.
People from outside of the US love to criticize how stupid Americans are for not having great buses and rail everywhere, but few want to acknowledge that no country is free of its own history and culture. Cars are a big part of ours. We are making progress, but the physical topology of cities and the culture loaded into the heads of several hundred million people cannot be refactored as easily as code.