Because these type of projects are almost indiscernible from organized crime rings. The rail companies strong arm the government into being forced to do it, through lobbying groups (why do you think this came about even though literally no one asked for it?) then they appoint all their buddies (the government appoints their people, the company appoints their people) to the project with massive salaries, and then at some point, the project is done (usually 10 or so years after) just look at the purple line extension, I mean how long has that been in progress, probably around 10 years. How hard can it be to build a subway? Now stretch that out over thousands of miles, plus the extra infrastructure for high speed.
What people don’t realize is, all the gov and the rail company’s work is over as soon as the contract is signed. One the money hits their bank, no one cares anymore.