The distance between major cities (the US has more than 50 cities with at least a million people alone) in many parts of the US is the distance between Paris and Rome, and they are pretty evenly distributed across the continent so you need mesh connectivity. In dense corridors that are closer together, they do use trains. The western half of the US is also extremely mountainous, which makes it a little more challenging to install a HSR mesh with many links approaching a thousand kilometers even though that is where some of the country's biggest cities are. Most of those links will never be economically justifiable given the amount of traffic that goes over them.

Because of the distances involved, air travel is much more economical in much of the US. Almost all of my trips to random cities in the US are at least a thousand kilometers; I think there is only one major city within a thousand kilometers, and that is connected by rail which people use.