Is that the fault of game design or players though? What stops them from fighting?
Ambition can establish reasonable cause for war. Even against allies. Claim a single region for one of your knights. State you'll accept peace after gaining that region.
That is absolutely players fault. They are scared to go to war with each other and get their realm destroyed. They worked too much to get their realms where they are now and they don want to risk it, but players are too proud to admit that.
