Amtrak can have high speed service to just about anywhere in the US that they want, they just have to pay for it, either through track usage agreements, or by spending the money to buy land and build a larger track and facility network than they currently own.
They rent usage rights from the rail owners on the majority of their route structure, and plenty of that track is not built for high speed rail. Also, in many cases the rental agreement gives them priority, but only in a certain window of their time schedule, so for it to work they have to be on time AND hope the freight trains don’t experience any delays on the main line.
High speed or even moderate speed rail in the US is a slopery slip… so to speak.