I'm going off of memory here, but what I heard was that passenger rail was never really as profitable as freight, so the government basically had to force all the rail companies to keep doing passenger trains. The massive subsidization of car transit made it hard for them to compete. Amtrak was basically created as a way to take the responsibility for doing passenger rail away from the rail companies, in exchange for guaranteed priority usage of the rails.

The priority of passenger trains is unenforced, so they have to wait for slow freight trains all the time. Additionally, the math on speed is different for freight vs passenger rail. A freight train getting somewhere an hour faster doesnt matter nearly as much as a passenger train getting somewhere an hour faster, so improvements to facilitate faster trains falls on amtrak.

Amtrak does own some rails, e.g., the northeast corridor, which is nice and profitable. It's also worth noting that the "unprofitable" lines are still vastly vastly vastly more profitable than roads.