What makes us think that most politicians get caught when they step over the line?

We wring our hands over politicians who get caught in illegal or immoral behavior. Yet there is no reason to think that those we hear about are the only ones. It is clear that there are many politicians who are not caught, but are guilty. Of course, that then begs the question of whether behavior which is licentious, but not illegal nor involved theft or bribery, necessarily hurts their ability to govern.