There are just more bad mixed breed breeders out there. Sure, some have nice websites and so on, but very few do health testing on the parents, or any sort of showing or events with the parents to know and prove the parents have good temperaments.
There are of course purebred breeders who are horrible too, you'd see THOSE pups in a shelter being dumped.
But there are also a lot of good, responsible, ethical breeders of purebred dogs. They do ask questions of the people who inquire about a pup. They do educate those people if they have screened them and decided they are a good home for one of the puppies. And they support them for life, so when there is an issue with the dog, the breeder is the first one called for help.
You wouldn't see those purebreds in a shelter, or even rehomed, because it's a rare situation that a pup they've bred has the issues that result in the dog getting dumped. I know many breeders like that and rehoming just isn't something that happens very often - and if it does, it's back to the breeder's house.