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The explanation is really simple.

If villains actually played the smartest game, the player would always lose, making for a rather retarded gaming experience.

It's much like the way a typical horror movie would be if all the characters/victims showed any common sense in taking the smartest action possible in a "horror film situation." You'd have either a very short (they all avoid the horror situation before the first act was over) or a very boring movie (nobody dies).