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It's not really that hard to get into the airline business. There are usually a couple of new (small) airlines every decade. Equipment is easy to lease, maintenance is easy to contract, staff is ok to find when you don't need a lot. You might not be able to get slots where you want, but there are a lot of airports, you can find slots somewhere, and you can do business with a handful of routes.

But there is a lot of competition; wikipedia lists 10 major (doing $1B annually) US 'mainline' passenger airlines, and three more major regionals (although one is a subsidiary of a mainline). It's hard to stay non-competitive with 11 other big companies doing the same things as you; especially when non-majors can compete too.

That's not to say there's no gouging, there is a lot of opaque pricing and nickle and diming. But, if your route is served by multiple carriers, there's probably not a lot of consistent profit there. If you're flying out of a small airport that does two flights a day and the nearest airport is a 4 hour drive, there may be some more profit there, but it depends on utilization.