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).







