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).
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).