Nobody
asked for Spirit Airlines.
I feel like they are a by-product of fare deregulation and the movement to self-directed online bookings. Like vermin before the invention of germ theory, they just appeared spontaneously on the market and nobody really planned it.
Deregulated fares eliminated the need and most of the opportunity for carriers to compete on experience. If PHX-ORD was the same price on every carrier, you're going to see all the marketing devoted to things like "nicest cabins, best food, most free amenities, most convenient schedule". That's a very hard message to maintain when 95% of customers start at a brand-neutral booking system and searches by price. I sort of wonder if they'd have succeeded in the age of agent-driven bookings, because agents would be disincentivized to recommend a carrier that's likely to yield bad reviews or loss of repeat business.
I think Southwest and JetBlue have done a moderate job of beating that cycle-- creating loyalty by promising "reliably slightly worse than awful". I know I'd preferentially book Southwest if I needed a flight just because I know I'm not going to be nickel-and-dimed with luggage fees.
Conversely, I don't think people's tastes for health care and education are as broad as you'd expect. There isn't anywhere near as much opportunity to "unbundle" features to try to hit a price target. Nobody says "well, if we can save $250 this year, let's not teach Timmy how to subtract", or "I want my surgery done by Steve, who just finished a three-week Coursera Tonsillectomy Bootcamp, rather than paying full price for Johns Hopkins."