I used to be surprised whenever news came out about a wealthy person committing insider trading or a priestly sex scandal or a congressional scam. These individuals are already at the top of their game with seemingly great jobs. Why risk it, why risk jail? The standard explanation is arrogance or greed. These people think they can do whatever they want and they get stupid.
That's a pretty sloppy explanation and I've come to the conclusion that the real reason is far darker: it actually happens all the time and you rarely get caught (or are rarely prosecuted). Indeed, Congress spends an average of $1 million a year for settlements of congressional employees. That's all taxpayer money, by the way.