It's a useful shorthand when you don't feel like bringing moral ambiguity into the discussion. This whole thread is predicated on the fantasy that there is some sort of absolute morality, or at least a somewhat sensible relative morality.The problem is that in D&D, Law and Chaos and Good and Evil (and even Neutrality) are tangible, objectively defined things.
It's hard to be morally ambiguous when gods are walking around literally embodying morality.
The real world doesn't work like that. It's why D&D is called "fantasy".