I suppose that's why the long campaign goals are only to acquire 40ish provinces out of the total 108. Maybe it would be a bit more sensible to conquer the whole map if other factions were more willing to become vassals once you are the pre-eminent power, even if they hate you. That way you could amass a huge empire on paper without actually having to physically conquer such a huge expanse. As I understand it this was how most empires were built.