Because objective morality makes campaigns easier.

Why are you killing these goblins?

Because they like eating kids. They're eeeeeevvvvilll. Don't think too hard about it.

Of course, the very idea of an adventurer band is based on roaming mercenary bands, and 1st Edition progression was just straight up tied to getting rich with murder. It's just a natural idea to question "Am I the baddy?" once your culture grows up out of Cold War "Greed is Good" bullshittery.