The problem is three fold.
1) The player learns, the AI doesn't. At some point, no matter how good or bad the AI is, you will find ways to always beat it. This is not something that will ever change (except maybe far in the future when even games have learning AI).
2) Paradox AI SUCKS and always has....
3) Games used to be FAR more complex and difficult.. "Normal" Difficulty back in the day was todays "Very Easy" mode.. YOu had to know everything about the game, and use its mechanics competently to be able to beat a game, or not struggle on normal mode.. (God I miss those days).
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This is why I still love CK2 far more than CK3 though. The game does and can screw you over in terms of abusive events back to back that just screw you over hard, and all your plants go out the window, forcing you to re evaluate the situation. Doesn't always happen, and yes you can still blob into a huge empire easily if you know the games mechanics, but those abusive back to back events, and string of bad luck in CK2 def keeps you on your toes, far more than CK3 ever will and keeps it interesting.
That's probably what CK3 is missing that you're looking for. INstead of the AI being the one to out do the player, let string of bad luck, and abusive events bring the player down to the AI level for a bit.