From a web developer's point of view, adding buddies, pm's, and the like within the game would make the forums redundant. Since the forums already provide that functionality, there is no need to include it within the game. I'm not even sure what benefits you would get from it other than being able to be lazy. Typing in someone's name for PvP is not extremely difficult, and having the forums encourages more interaction than you'd get if the social aspect was limited to only within the game.