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).
Population density is not really a problem: Most of the country is virtually empty, but there are a series of urban agglomerations that have incredibly high density overall. The North-East corridor (DC to Boston, more or less) is the most obvious one, but Chicago and environment or Coastal California are great options, too.
The “secret” reason why it’s not happening is public indifference and corporate sabotage. A campaign of decades of worsening public transportation has made people convinced that a high speed train would be just for poor people, which they imagine to be someone else. Also, eminent domain land seizures are slow, environmental impact studies slower, and both force costly changes from original plan that the public hears about as cost-overruns.
Final nail in the coffin: in America, for bizarre reasons, passenger rail has lower priority than freight rail. The freight rail companies don’t want to give up the privilege, and obviously you can’t have a high speed service wait on freight trains bumbling by.
Every poor american is a temporarily inconvenieced millionaire, according to their own mindset.
The reason for the freight priority is because freight companies own the rails and contractually de-prioritize Amtrak