Because not enough people live along the way. Sure you an build track and run trains, but 5 hours on a train is about the time where flying is enough faster that people will fly instead of taking the train. Less than 5 hours train competes well (stations are closer to you, and no long security lines), but after that airplanes are enough faster that few people would use a train. That means only a small number of people will ride the train for those middle stations.
Sure if you are building a track you can put in stations in towns that don't generation much traffic, but you still need traffic from somewhere and it won't come.
Well with Maglev, Portland to San Francisco could make sense via Sacramento. You wouldn’t want to do it on the coast because of the mountains, and there’s at least Redding and Ashland and a couple of other places in-between.
At 500 km/h (310 mph) you could feasibly do Sacramento to Portland in under two hours. The less straightforward question is how much of the metro area do you serve around those two cities, and do you connect that line directly to San Francisco or do you run a separate line to Sacramento via the Delta? Do you go a sort if L-shape around Stockton first? The politics of this could push travel time up, but at 500 km/h you can cover a lot of ground, much of it fairly empty.
So a hypothetical Best Coast system would connect Vancouver, BC to Portland, Portland to Sacramento, Sacramento to Reno, Reno to Las Vegas, Sacramento to LA, San Francisco to Sacramento via Stockton, San Francisco to LA, LA to Tijuana via San Diego, LA to Las Vegas, LA to El Paso via Phoenix, Phoenix to St. George, Las Vegas to Salt Lake City via St. George, San Diego to El Paso via Tucson & Mexicali and now you’re in Texas where options include El Paso to New Orleans via either Austin & Houston or San Antonio & Houston, Brownsville to McAllen, Brownsville to Houston via Corpus Christi, Corpus Christi to San Antonio, Houston to Dallas, Dallas to Oklahoma City and I’m probably missing some, but you have the workings of a Gulf Coast constellation anchored by Texas on one end and Florida on the other.
Thing is, I’ve worked this all out on paper too, including a Northeast, Southeast and Midwest map that looked much like one that someone linked to up the thread. Problem is, our choice of infrastructure is downstream from our cultural preferences which in turn are shaped by the infrastructure our ancestors built in the decades prior.
El Paso is about 725 miles away from San Diego according to Siri, so with the best trains in the world you could be inside the Texas rail constellation I briefly outlined above in about 2-3 hours which in turn could serve as the basis for a Gulf Coast constellation connecting Florida and the Southeast connecting to the Midwest and the Northeast and then to Canada.
It’s not built though because people just fly instead. We worked out how to get cheap air travel long before we figured out super-fast transcontinental rail travel that probably doesn’t make sense coast to coast but if it already existed, probably would make sense going coast to middle and middle to middle and would just happen to connect the coasts. Or you could just fly, which is what we do, and since that already exists and is even faster than rail travel, metro areas can figure out how their own inter-urban rail systems that are slow and local where it makes sense to build them and just try to make sure the airport is connected too. I like trains, but not so much that I’m willing to toss hundreds of billions of public money into some kind of “national rail system” whatever form it took for whatever prestige it might bring. Jumbo jets are cool too.
Wind resistance at ground level means that an airplane is much more energy efficient at those distances and speed. As such those distances are unlike to work just because an airplane is so much cheaper even if because of speed it is time competitive.
Sure if you are building a track you can put in stations in towns that don't generation much traffic, but you still need traffic from somewhere and it won't come.