Attributing conversions to ads is a really difficult problem. There's probably a decent amount of cluelessness and institutional inertia that keeps them spending on ineffective ads. See the recent stories on Uber drastically reducing ad spend without seeing any downstream effects.
That's the dumb ML practice of those companies, not Facebook. Also even an irrelevant ad probably converts through brand exposure alone. They are forced to fork over to compete for limited exposure.
FB adds told them to. The true end game.
Some are just finding out now that FB ads aren't effective.