I would guess because Amtrak isn’t being subsidized enough. A lot of government money is spent on building and maintaining roads. If the consumer had to pay for that directly in the form of toll roads instead of through taxes then Amtrak would be much more competitive.
I believe part of it too is that much of the rail infrastructure is owned by companies, and so Amtrak is getting permission from them to use their tracks. Freight trains get priority over Amtrak trains, for example, and I assume Amtrak has to pay for permission to use those rails.