Once you get wherever you wanted via train, you are now pretty much stuck at a train station.
So much of America is not walkable, and we don't have great public transportation within cities and towns, so if you arrive somewhere without a car there's not much for you to do.
But the same is true of flying. Once you get wherever you wanted via a plane, you are pretty much stuck at an airport.
The difference, of course, is that airports tend to have sufficient passenger numbers to justify car hire agencies locating there (or at least providing a shuttle), but that case is harder to make as long as there are comparably few trains per day.
So much of America is not walkable, and we don't have great public transportation within cities and towns, so if you arrive somewhere without a car there's not much for you to do.
edit: of course, ride sharing helps.