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).
California was working on a bullet train (California is pretty big, bigger than Japan). The politics killed it and then they ran out of money, so they gave up.
The US can barely keep bridges from falling apart. The roads are equally as bad. But we can definitely spend money on ball rooms and bombs.