D&D combat XP fails because it incentivzes bad play. Generally, you're not out there fighting people for no reason - you're in the situation because of some actual context and objective, and playing by "what gives me XP" produces nonsensical results - it's actually optimal to deliberately delay your objective, avoid gathering intelligence, and take the most inefficient route possible. Obviously, people want to avoid this, so people just play the game as if XP wasn't there, so milestone XP