Cheating is most effective when they have somebody at the table helping them get away with it. Their partner may not even be aware they are aiding the shark in plying their trade.
Who wouldn't want to be Gus Hansen? Bald, Danish, and completely unreadable at the poker table. Gus is called the "Poker Trashman" because he plays with junk hands as often as he plays with premium ones - and wins.