The US is also densely populated; when people are talking about high speed rail they are talking about connecting the major, close by metropolitan areas that most people live in.

The Midwest, as an example, has roughly the same size and population as France with a larger economy. In fact, if you overlay the French TGV network onto the Midwest with Chicago where Paris is, you get a pretty good approximation of where major Midwestern cities are located: https://www.reddit.com/r/MapPorn/s/NMr3J3gt8C