Related to Mook Chivalry, except it applies to all villains. The villains will never go for a sneak attack that is really any good. Some things include:
Killing the hero when he's asleep is dishonorable — A villain would never have the common sense to simply sneak (or teleport) up to the hero while he's asleep. Common sense would tell you that a villain could easily win by waiting until the hero [...] [tvtropes.org]