Where there is one-train (or less) each way per day, Amtrak has to run on the maintained tracks of host railroads. Taxpayers aren't willing to spend "a couple hundred million dollars" to acquire 140 miles trackage to support limited use by Amtrak, (note: SWC through western Kansas, CO and northern NM). It's not about "state govenment didn't give a damn" The solution is a dedicated PHX-MRC shuttle.
???? "One train or less" has nothing to do with whether or not Amtrak owns the track. There are plently of locations where Amtrak runs more than one train each way a day on tracks that they do not own. Ignoring the lines hosting state supported trains and there are several of those, the CSX line between Washington DC and Miami is the first one that comes to mind.
BNSF owns the most of the rest of the route used by the Southwest Chief, not just the parts where there is very little other traffic on the line.