Short answer: cities are too far apart and the USA is large. However, how much funding is there to really implement the same thing that exists in Japan but in the United States? Also, is there an incentive for that in the first place? What about population density? Japan is more compact regarding their population density while that’s not the case for America plus both Osaka & Kyoto aren’t too far from each other (but Miami & Washington DC are distant).
It’s less about size and more about population density.
Japan is 338 people per square kilometer https://en.wikipedia.org/wiki/Demographics_of_Japan
China is 147 people per square kilometer total https://en.wikipedia.org/wiki/Demographics_of_China
The US is 36 people per square kilometer https://www.census.gov/data/tables/time-series/dec/density-data-text.html
The US could build long distance high speed rail like Japan and China, but the ridership level would be rock bottom due to the low population density.
Those statistics are misleading. China’s population is very unevenly distributed between the east and west, but Western China is still serviced by high-speed railway.
The 1786km Lanzhou–Urumqi HSR serves three Western Chinese provinces: Gansu (57.7 people per sq km), Qinghai (8.2) and Xinjiang (15.8).
Do the densly populated areas should be first to be connected im the US.
I think it would be fine to just build the stations in big cities. Nobody is demanding high speed rail across Alaska.