You have to remember how big the USA is. About 3.8 million square miles. With only 321 million people to pay for these bullet trains. (They will need new tracks so they don't run into slow trains) Compare this to Japan, which has only 146,000 square miles and about 127 million people. The length of the tracks in Japan is extremely small compared to what USA would need, and more people would use it. How many people would travel on train from say Huston to Las Angeles every day? Enough to pay for all the tracks etc?
And China has them also, but much larger population to travel on them, Plus I don't believe they have the highways the US has and must maintain.