Because nobody knows how to build them. The concept of building something well is alien to large corporations here in the 21st century.
............Bottom line is that even though a rail car could look like any fine hotel, the powers that be no longer have the slightest interest in running trains like that. So the simplest, to clean, and take care of is all your probably ever going to see again.. But it is nice to dream!
The reality is that it is not 1956 anymore - with no intersates or jet airplanes, when the average person rode trains as a normal course.
Your mostly correct on the dates! Beyond that it still takes the same amount of time, (well actually more), to travel the same routes. So the public still needs a diner, a decent set of sleepers and coachs, and some place comfortable to relax and enjoy the scenery and fellow passengers.. None of that has changed a bit. The concept developed in 1956 was to attract passengers with a good reason to purchase a rail ticket rather than the fast growing air system. Yes the air won, but the pendulum may be swinging once again and the same attractions make rail a possible alternative.
You can run bare bones trains as Amtrak does now, and it will have riders who have choosen not to fly. But if your going to get people to spend two to three days on a train then the consist that makes the trip exciting becomes more of a good business idea.
That is the one place that competition did for at least a while play a part. When your running the only route to california from chicago you have no need to try and offer a better product to attract passengers.