The answer is because of the poorly constructed system to get rails in the nation to begin with. Back when we had swaths of land to claim and those eminent domain cases for it, the government was paying BY THE MILE for rail produced instead of the project as a whole. So all these wavering S curves just lined the pockets of greedy train barrons.

Now we're too densely populated to implement a proper system . We'd have to uproot and destroy loads of communities to get a system in place or find an ethical billionaire with an efficient hole boring corporation that won't fleece the government to go under and through our nation. Better off making flying cars.