Because Spirit's baggage fees are so high that it'll more than make up the difference in ticket prices. One reason why I prefer Southwest so much is because I can get both my carry-ons and two checked bags with the ticket price... which is still lower than any of the legacy carriers.
(which is a reason why I'm going to be so upset when the MAX is put back in service and I won't be able to fly Southwest anymore)
If I'm staying for less than 4 days I can fit everything I need in one backpack. With a backpack and a carry-on I can bring everything I'd want for weeks.
* My purse counts as a carryon. I don't go anywhere without it.
* My other carryon is my backpack, which contains my electronics. I have my Surface, my headphones (which are very large padded cup models... I have sensitive ears and can't wear earbuds or small hard things), and a variety of chargers and spare batteries. I intend to use at least some of these on the flight itself. And I also stuff the side pockets with snacks for me to munch on during the flight (usually red/yellow stuffed Twizzlers).
* I have a suitcase for my clothing. I don't really like wearing the same clothes every day, so yes I pack multiple changes of clothes.
* I have a duffel bag for all my toiletries. Makeup, shampoo & conditioner, hand cream (I tend to travel in late fall/winter, and the cold fucks up the skin on the backs of my hands for some reason), shaving equipment, pads, flat iron, etc. Some of these can't go into a carryon because of bullshit TSA rules.
And on one of the trips I made last year, I also brought a number of toys for my niece and nephews (three kids, two toys for each kid, and not all of them were terribly small), which I had to basically play Tetris with to get them to fit in my luggage. I think they ended up being distributed between my backpack, suitcase, and duffel bag.
* Eight shirts (4 undershirts, 4 overshirts)
* 4 boxer-briefs
* 4 pairs of socks
* 1 toiletry kit
* 1 laptop or tablet depending on the nature of the trip
* 1 phone charger
* 1 tablet/laptop charger
* 1 medium sized book for the plane/airport
On my person I carry a wallet, car/house keys, airpods, and cellphone.
As for southwest, it depends on timing. They are cheap early on when major carriers are asking standard prices. Then as you get closer, southwest cheap seats fill up while major carriers start cutting prices, sometimes drastically if it turns out to be a bad travel season.
Traveling with four bags is too much of a pain even when it's free to bring them.
It that on principle, or fear of safety?