Amtrak started out as a holding company for private passenger rail companies that went bankrupt. It's never had a static amount of funding (until the Biden admin, Amtrak had to renegotiate its budget regularly) and many of its stations are just pet projects for rural Congress reps who want to give their district a way to leave their area, so Amtrak runs many trains at a loss.

Building new rail projects in the US is very hard because of capital costs and regulations like NEPA (and CEQA in California) which require environmental review for everything. Brightline in Florida was able to get around this by working in an existing highway ROW.