Well the same argument can be made about cops planting evidence, yet we have seen them do it on camera. Several similar examples, from politicians to people running ponzi schemes- despite the fact its not mathematically possible for one to continue for very long. Most humans, almost always incorrectly, think they are smart enough to get away with it. So they often don't weigh the pros/cons as logically as you just did. I very much doubt that anything humans do that is supposed to be random, is actually random. Whether that's drug testing, security checks, DUI checkpoints, etc.