When evaluating the price, consider that it is also a hotel room and comes with free meals. Compare it to air fare + hotel room for X days + decent meals for X days and the price becomes less painful.
That would be 3 days on Amtrak, or about 6 hours by plane.
I'd guess that for most cases flying instead of taking the train doesn't mean we spend the 6 days that we would have spent on the train (round trip) in a Los Angeles hotel eating expensive restaurant meals. I think most people would simply leave home later and leave Los Angeles earlier if flying, so that the time difference is spent at home eating whatever they normally eat.
Note that some people use Amtrak because they are afraid of flying. This way of looking at the prices does not apply to them.
Personally, I sort of want to do at least a section of the Empire Builder or the California Zephyr at some point. And I'd also like to take a transatlantic liner (well, the Queen Mary 2) from NY to Southampton. But probably only once and I'm fully prepared for the idea to be more enticing than the reality.