I wouldn't consider a one seat ride over 2000 miles (or two nights) to be reasonable. Now a one roomette ride, sure.
The TE between CHI and LAX is close to three days long. I personally would never travel between the two cities on the TE when the SWC exists although I know people at AU have said they have and insist that the TE should exist for that purpose. Now would anyone here would ride the whole way in a coach seat I don't know. But the idea that a three day train is a deal breaker is a crock when the CHI-LAX TE exists. You can say equipment isn't available for a NYC-SF train but don't tell me no one would ride a train for three days in a row without getting off.
The TE/Sunset doesn't just exist and people aren't taking it just because it's a three night ride from LA to Chicago. And I don't think anyone in AU has said it 'should exist just for that purpose'. But many people (like me) like the change in scenery and want a route that goes through the south. Nobody is taking an LD train to save time. They either do it because they have to (maybe flying or driving wasn't an option), or because they like to spend time on a train. For you, Amtrak is just how you get from point A to point B. But that's not how it is for everyone. In fact, here were plenty of coach passengers when I was taking #422 that were going the whole way (or almost the whole way) so clearly they must have had some reason not to take the Chief.
And while an SF-NYC train could also be three nights, it would be at least 20 hours longer, and would require significantly more equipment and operating costs. So it's not just that a coast-to-coast train would be three nights ('cause it probably would be four in practice), it's that it would be the longest route in North America, and the most difficult to operate, and with some of the least demand. That's why.