This is a matter of geography. Your main eastern market is the DC-Boston corridor--as populated as all of the other states on the Atlantic seaboard combined. So that means that you're mostly looking at terminating there, no matter where on the West Coast you originate. If you start in the Northwest, you have to cross south of Lake Michigan to reach the NEC, so Chicago is en route. If you're in Northern California, you'll emerge from the Front Range near Denver, and you can pick whichever hub you want--might as well pick the one everyone else is using and choose Chicago. Even as far south as Oklahoma City, going to DC via Chicago only adds 100-200 miles to your route. It's not until you get to Dallas that reaching the NEC via Atlanta/Charlotte/Raleigh-Durham makes sense.
Historically, by the latter half of the 1800s, the railroad industry already settled on using Chicago as the main termination of every major railroad, and thus the major transshipment point. Geographically, the logical point is bounded by the southern tip of Lake Michigan and northern bank of the Ohio River (as you have prairies to the north and hills to the south). In principle, a city like St. Louis or Indianapolis could have grown up to be the major transshipment hub instead of Chicago, but you'd still be complaining about the lack of southern East-West lines in that scenario.
It's even crazier than that. It turns out that Amtrak would be profitable were it not for being required to run long distance passenger rail lines through sparsely populated states, usually represented by the senators you mentioned.
(Note that regional state co-financed Amtrak lines like the Capital Corridor in California aren't part of the profitability problem)
The same senators from those low population density states who demand that Amtrak cut losses in order to become profitable also raise hackles when the obvious solution of cutting the unprofitable long-distance routes through their states routes is raised. It turns out that many of their constituents like those unprofitable routes.
See Wendover's YouTube video on the topic: https://youtu.be/dSw7fWCrDk0
https://en.wikipedia.org/wiki/Virgin_Trains_USA
Lobbiests for commercial interests who are potentially undermined by effective rail have long managed legal strangleholds on significantly improved rail infrastructure.
A glaring case in point, all passenger rails connecting East and West must go through Chicago -- far north of southern states. (There is one claimed southern line, but it never appears to be available for transport.)
Worse still, the Republican party has bamboozled their members into actually believing trains are not cost effective and therefore just a give-away to the undeserving lazy jobless welfare recipients who use them, because they can't afford cars.
Yes, it's that fracked up here.