People tend to organize things into dichotomies for easier cognitive conceptualization. Black/White, Good/Bad, Up/Down, Hate/Love, Male/Female, War/Peace. When in reality, the only true dichotomy is Life/Death (and even that will probably only get less and less clear as our understandings of things increase).
But it's just so much easier to say, "Oh, here are the good guys and here are the bad guys", in one's head, than to say, "Well, Group A and Group B share many similarities, actually, and their conflict largely revolves around mutually-conflicting self-interest-based competition for scarce resources, complicated by the ambitions of the various individuals in leadership positions and sub-factions within each faction vying for internal influence and their own personal goals".
One of my favorite things about WoW is how realistic the politics are. Or at least reality-influenced, rather than the fiction staples of "good vs evil". Even the non-Horde/Alliance forces are not completely clear-cut (although you might have to dig outside of the game itself for these perspectives -- but I've read some interesting theories regarding the motivations behind the Burning Legion / Titans / Old Gods).