Because war is inherently a bad thing. Only shitheels and people that profit from war (read: shitheels) want it to continue for the sake of being at war. If players want to have their evil power trip fantasy that's fine, but for this game to have a remotely believable story then the people who seek war have to eventually either cool their shit out or get put down. Otherwise they'd just get dethroned by their people who are tired of being thrown into a meat grinder for their leaders' personal grudges.