Another theory: Once this pattern is present, it's self-reinforcing in the same way that "good guys wear white hats, bad guys wear black hats" was in old Western movies. Most of the audience knows the meaning - and if you're making the audience think too hard, then your old Western or action movie is likely to flop.
Cool so I can build suspense/tension by naming my hero John and my villain Jack, the audience won't have a clue until the final showdown!