d&d gives you a lot of ways to resolve combat...but if there's no combat, there's no d&d. there are tabletop games where you get XP for character development and interactions, and accomplishing missions where there was never any intention of killing something or being involved in "combat" and if combat does occur, you're more likely to be docked XP than awarded it. this is why d&d is usually referred to as a combat heavy game.
to be fair, combat heavy is not the same thing as hack and slash. if you've got a good crew, chances are you will resolve a lot of conflict through diplomacy, or charming spells, or attempts at seduction, stealth, deception, bluffing, etc. but the point is, if your bluff check fails, the guards will aggro you, and then you'll kill them and get the same XP as if you had successfully bluffed. getting xp for character development, plot resolution etc. is of course up to the GM, but there's nothing in the d&d rulebook that says such a thing HAS to be done, whereas there are systems where there are rules for plot or character based xp.