It's a trade out. Usually Southwest is the cheapest (but not always) and no charge for luggage. The trade out is their boarding system.
Southwest makes no secret about it, the number one complaint they get is their boarding system. They did try assigned seats for a while a few years ago out of San Diego, but ended it.
Sort of like McDonalds, people love to talk smack about them, but people still go there. I live a mile from a row of fast food restaurants, and 2 Saturday's ago I decided I wanted a Double Cheeseburger. Couldn't get near McDonalds, every parking spot filled, the two line drive up had cars backed up. I ended up at Burger King a block away, not a single customer inside, and 1 car in the drive through.