coyote6
Adventurer
This doesn't always hold true everywhere in the states, but for the most part, busses are seen as cheap/poor transportation.
Some places & times, busses aren't terribly reliable or convenient, for in-town use. I used the bus to get to work in San Jose for a year or so, and I had to leave 30-45+ minutes earlier than I would have with a car, because the bus took longer (frequent stops), and the schedule was such that I had a choice: early to work or late. Then there were the times the bus just didn't show up. And I had a direct route, too -- no changing lines or anything. It was as simple as could be.
For cross country trips, a bus is usually slower than driving yourself -- and if you drive, you control when and where you stop, don't have to hassle as much with luggage, etc.
With plane trips, you also get modern American airport security, which can be a drag.