> Once you're on the plane and its in the air, Spirit and Frontier are like pretty much every other domestic airline.

Yes, if you ignore the part where things are different, it's basically the same. Trouble is, those differences do meaningfully make a difference. There's no objective measure for misery and happiness, but flying Jsx is nicer than Spirit. You can put a dollar value on misery, that's why one's so much more expensive than the other.