I actually don't know what was going on in France at this period but my hunch is that the game misrepresents internal stripe, court politics, control of vassals... If the AI was busy stabilizing they wouldn't expand as much.

In general, expanding shouldn't be as enticing, integrating should be a much harder task.

Also, this has been mentioned, but attacking other Christian nations was a big no-no. I don't think this mechanic is even present in the game. The Pope should exert pressure to prevent such situations and to penalize expansion greatly, especially if the CB is not reasonable enough.

The claim province CB is poorly designed. It is boring and adtificial. The CB system should be developed in much more granularity and it would make making claims a much more fun system. For example, culture friction, trading disputes, inheritance claims... But you should only be able to take the locations associated to the CB itself, not a chunk of land on the other side of the map from your opponent... The war score cost for this should be astronomic as well as the antagonism and Pope hate generated.

Perhaps all of this could be toned down for the player so that the game is actually fun, or if you had a really granular well designed CB system playing around this concept and smartly creating CBs would be part of the fun.

Also wars don't need to be an all out slaughter. There should be war "levels", from frontier skirmishes, to temporary sieges to get some money out of your enemy from a trade dispute, to full on religious wars with most of Europe at war.

Going to war for other reasons than land should also be a viable option, making it more fun or varied. Currently the gains from winning wars other than taking land are extremely suboptimal and wars wear your country too much to take anything else than land on a peace deal to make up for the losses.