That's the thing though, they rely so much on creating the inheritance issues that it massively overinflates the gulf between players and AI. Players can manage their inheritances via proper preemptive work. As a result, it's fairly easy for a player to maintain their domains and realm cohesion. What's more, on the off-chance that a kingdom title splits off, the player is capable of working toward retaking it as a long term goal. (or immediately if you managed your domains correctly, and don't have huge factions to contend with) In the end, these 2 facts make it incredibly easy for the player to reach empire level, at which point it just becomes a matter of creating duchy/kingdom titles to hand to your kids. The AI on the other hand is a lot more scattershot. As far as I can tell, it doesn't preempt the inheritance question in any way, leaving children to just get whatever they would naturally get. Any AI country that miraculously gets larger than 40 provinces (halfway to empire) is always 1 bad inheritance away from complete dissolution, either by split kingdom titles or revolution. Worse, when this does come to pass, the AI doesn't make any noticeable effort to reclaim lost ground. I rarely see AI's that split into several states via inheritance actively pursue reunification.
So what's the end result of this? The player is the only person in the world who is actually capable of forming an empire-sized country prior to primogeniture. (which comes too late to matter) And by the time you reach primogeniture you likely dominate an entire quadrant of the world map even if you weren't trying to. This of course has the knock-on effect of trivializing most of the game past 1-200 years because just being an empire automatically makes you twice as large as any possible country you could conceivably deal with.