This is a fairly common sentiment, often apposed by those without children, which misses the point.
Yes, parents want to be able to make sure their kids don't watch things they don't want them to see. They want to pay attention to what their kids are watching. The problem comes when there is no guaranty that any given show won't contain 'bad' things, thus they cannot make as intelligent a choice* unless they watch every single show all the way through before they show it to their children. And parents who care still don't have infinite time on their hands.
*Yes some shows can be ruled out immediately, but others could have things you wouldn't think they would.