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).
The difference everyone always ignores is that most of Chinese infrastructure is new. For the US they’d need to buy people out of their land, build new tunnels and bridges, and disrupt so many things to implement high speed rail.
You can’t just leverage the existing rail network because they have curves and grades that are incompatible with high speed rail (northeast corridor has 30-40mph limits on some curves).
In addition you’re competing with airplanes which are already proven and support current travel demands. And even if you could get the rail implemented there isn’t any guarantee it’ll be any cheaper than flight (meaning low usage). As it stands today going from major hub on Amtrak can be more expensive and takes an equal amount of time (when accounting for security/etc.)