From what I hear, people think they have the perfect system to steal or embezzle. And they may get away with some small amounts of theft.
I’ve worked with people who investigate these sorts of crimes. And a common denominator, is that the perpetrators eventually get greedy, steal larger and larger amounts, and think they have it all figured out, and that they will evade internal controls and audits.
Generally, they start out small, and think because they got away with it a few times, they can get away with it for bigger amounts. Eventually they get caught.