It is because it is a turn based game, i would assume? So they need to give time for everyone to finish up their turns?
I will tell you, its not the worst i have seen in a game..... i like to play a game called Medieval 2 Total war.....
If you are a Catholic nation, at war with another Catholic nation (AI) Then you are subject to decrees of peace from the Pope. (You actually can refuse but its not good for various reasons i wont go into).
The AI uses the Papal peace treaty to advance its territory and block you from advancing on it.... its akin to the AI taking your worlds in Calciv3 and using peace treaties to stop you taking them back!!!
This is how it works.... the AI goes to war with you, you beat off its attack and besiege one of its cities. It then calls for a Papal peace treaty (the player cannot do this). You are forced to break off the siege. Then that same AI will prepare another army and besiege one of your cities again, you cannot call for a piece treaty like the AI can, so it may take your city, then when you send an army to retake your city, the AI can call for another piece treaty and stop you. Then it repeats the same pattern.
Here and there the AI can even get away with disobeying the Papal peace treaty seemingly without consequence.... that one i could never work out? Other times it does get itself ex-communicated. But the AI will magically be un-excommunicated as soon as you take one of its cities and the peace treaties will start coming again.
Of course there are ways to cope with this peace treaty cheese for the AI, but the way it works is the most amazingly unfair system i have ever seen in any game ever! lol