People didn't go on rampages and take over entire kingdoms because they felt like it. There was a very strict feudal system in place that was rigidly enforced by virtually every power structure in Europe. While one mad king might want to just rampage through the bordering kingdom, nobody would support their claim and they would have a hell of a time legitimizing their ownership of those lands.
Think about it, you're a duke in a kingdom. You have your lands to look after. You have your armies that you want to use for your benefit. Dukes only grudgingly granted their lieges control of their armies, and only because they respect the laws of the kingdom and the feudal structure. This is decentralized power, kings generally don't have the power to throw their entire kingdom at the meat grinder just because they're feeling extra bloodthirsty this morning. Don't you think it's more likely the dukes would just refuse? Or even depose the king?
So while it's certainly possible that one mad king might convince his dukes to go to war for no reason, and it's certainly possible that the dukes might convince their counts to go to war for no reason, it's all so unlikely and far-fetched that putting it in the game would just undermine the elegance and complexity of how stuff actually got done. Ie, through marriage, interpretation of laws, intrigue, plotting, and the inevitable war to back up those machinations.
