Because there aren't rules for Social Combat.Why don't you take the same approach to social situations?
If there were rules for Social Combat like there are for Weapon/Spell Combat, then we'd follow those rules as written. And the same way a player can't just state a "cool description" of their attack and gain a mechanical advantage per the Combat rules... the player couldn't just say a "pithy quip" and gain a mechanical advantage in Social Combat either.