1. Many heroes and villains are hopelessly overpowered. Thus they have to be a jobber most of the time so that people won't lose interest in them (I'm looking at you S-man).
2. The most successful or feared and admired villains rarely job, if ever. For example, Luthor never jobs (not that I've seen), he seems like he's trying his hardest every time; not pulling any punches like others do.