That's a choice. Maybe not a good one. But, still a choice. Not all choices in life are optimal. If enough people said "I refuse to board another United jet ever again" their customer service would improve. But, by continuing to fly them because the "other" choices are worse, they have no incentive to change their service. They are "doing enough" to keep you on board.
I guess you can't entirely blame United and Spirit for their poor customer service. I think their bean counters have told them that customer service doesn't matter. Too many customers will search for airline fares and even if United is $1 cheaper than Delta they will book United.