A basic reason that people break rules is that they believe they can get away with it. And you're more likely to think that you can cheat without consequence if you know that watchdogs won't bark, much less bite.
This observation doesn't just jive intuitively with our understanding of human nature, it also squares with lived reality. Fewer police means more crime. No drug testing in baseball means an epidemic of steroid use. Rare audits mean more tax cheating. And so on.