It really depends on the route. Personally, I live in Chicago, which means I have lots of options going to midsized and large airports, and prices can be very competitive.⁰ Small airports are often served only by one of the three airlines with a large presence here (United, American, Southwest), and it's these routes that have higher prices. In the past, I've taken 13 hour (overnight) Megabus trips instead of <2 hour plane trips just because United's prices on some small monopoly route or another were so outrageous. For those who live in a town or city with few options like that, airline travel is a lot more expensive than it is for us lucky ones in hub cities, takes a lot longer because of transfers, and is a lot less flexible with flight times and such. This is a factor behind a lot of corporate HQ relocations to big cities, as well.

⁰ I actually recently bought a roundtrip to Denver for $32 fully inclusive of taxes and fees. Granted, it's on Spirit, which is only marginally better than the bus, but can't beat the price.