Hussar
Legend
It's a good question. But I think the answer is because of all the fuzzy stuff we been saying here.
STR is easily managed by the number. You do or do not get a +5 to your attack and damage. You do or you do not open the door. The player's strength has no impact on the character's strength.
INT might convey some numerical bonuses, but in terms of defining how smart the PC is, it still relies heavily on the player's brains.
CHA is equally fuzzy. There may be rules on how many henchmen I get, or some impact to social skill checks, but like intelligence it still relies heavily on how the player portrays it.
WIS is the same thing. Your PC might have a high score, but if played by an impulsive person who never makes a smart move and always falls for every trick, then you don't have parity.
That's the point. You can't easily align the attribute rules to the player's ability because its not something the game can fully contain or restrain.
But, that's my point. I shouldn't have to adjudicate this. The player, him/herself should be doing so. The player should be playing the character that was created, not ignoring any weaknesses simply because he can and the DM lets him.
In other words, it's always bad to not actually play your character. If your character has a 10 Cha and no ranks in diplomacy, then, as a player, that's what should be getting played. As DM's, we should never applaud players who ignore their character sheet.
The funny thing is, in any other part of the game, ignoring the character sheet is called cheating and we consider players who do this to be very, very poor players. Yet, we've seen numerous examples here of DM's who not only don't call out players who don't play the character that was created, but actually actively reward and applaud these players as bringing the goodness to the table.