Also, there are many things that I can judge to be good or bad, but that doesn't mean I can produce high quality examples myself. And getting high quality work

Also, there are many things that I can judge to be good or bad, but that doesn't mean I can produce high quality examples myself. And getting high quality work from others isn't cheap, so it might be out of reach for self-published authors. Even if they know that the cover art isn't good, they might still use it, if they can't do better themselves and can't afford to hire someone who would do better.