Poor umpire knowledge. All some umpires want is the pay. The heck with wasting my time on any rules training.
While I agree that there are umpires with less than stellar rules knowledge, I do not know of anyone who is doing it just for the pay. You tend to find umpires with poor rules knowledge/improper mechanics at the lower levels. The pay at this level is hardly enough to cover costs that umpires incur so it would be hard to say it is a major part of their income.