Like the other comment said. There's also a time element. Who wants to take four flights, get stuck in an airport, and possible lost luggage when Spirit has a direct flight.

Precisely. You won’t catch me dead on a Spirit/Frontier flight because I can afford more comfortable/reliable options for the length of flight, but I am not the target demographic. Having cheap options is good in any industry. I fly budget airlines in Europe quite a bit because the $ saved per ounce of discomfort ratio is far higher and am thankful they exist. One’s inability to read the conditions of their fare is not a valid reason to eliminate the option for everyone else.