I think one of the other big issues on why people would rather fly is simply time. They would much rather go thru all the humiliation, stripping, lines, etc because it'll only be a couple hours on a flight and then they are at the airport where they need to be. Rather then spend a couple days enjoying a relaxing trip across the country by train.
I hate what happens in airports these days and I would completely avoid it if I could, but it's not that simple for many of us. Try telling your boss that you need an extra two or three days of travel time
each way for double the cost of a plane ticket. Even when I'm just visiting relatives I can either spend two days with them and two days on the train or four days with them by plane. Which would you choose? Taking the train is a sacrifice for many of us and just because other folks choose the plane doesn't mean that they don't mind being groped or irradiated.