Because they are taking away from the guest experience! At least for the majority of the guests.... those that are on budgets!
And most of the other companies are trying to do the opposite... increase the guest experience at the same time as increasing prices.
I've worked fairly high up in both banking and more conventional retail and I can say pretty unequivocally that that last bit just isn't true. The principal goal in American customer service for at least the last five years has been to systematically reduce the client experience in an attempt to find the absolute worst experience you can provide without it affecting the top line. Client experience is expensive. So if you can reduce it without affecting the top line you grow your bottom line. I've never worked for an airline but if you honestly don't feel like your airline is doing that I want to fly them!
To be honest one of the things that really struck me about Disney was that it didn't seem like they were doing that, but rather were content to simply provide a genuinely premium product at a premium price.