The impression I'm getting is that the Alliance is either fighting to end the war, because war is bad m'kay or that the Alliance is fighting the Horde for the sheer hell of it. There is no end game plan or goal for the Alliance. No one talks about destroying the Horde (lots of talk about fighting it), at best there's talk about removing Garrosh from power, but no one has said why he's actually bad.
So why doesn't the Alliance just surrender? If they don't want to fight the war, then they should give up. If they don't want to destroy the Horde, then they should give up. If they don't want the lands the Horde has taken, then they should give up. If they don't want Garrosh to be a jerk to them, then they should stop atagonising him and he'd treat them better.
The Alliance talks alot about how you fight. They don't talk alot about why they are. The players are left to imagine why, but the only reason supported in-game is that Garrosh is simply intolerable (for whatever reason) or that you're a crazy belligerent psycho that just wants to hurt the Horde as much as possible for no real reason.
Even if you're convinced that being ruled by a Warchief is completely horrible existence, can you really say it'd be worse than fighting a war that spans the globe?