The AI get a variety of cheats both campaign and in battles. The higher you go, the worse it gets. I just play on Hard nowadays as i do not find the challenge above that to be fun. Not to mention it removes options from the game as to what you can do.
it's quite hard to do a more intelligent ai or a more stupid one, so they just solved it with buffs for the ai.
like on legendary difficulty the ai doesnt pay for colonisation, can see the entire map (except armies in hidden stance), has 80%less upkeep, 70% less recruitment cost, +125 growth per turn, +500xp per turn, -80% public order penalty, way higher unit stats and so on.
so you could fight a 1v1 against the ai with the exact same unit and you would lose because your stats are worse.
and i haven't even started with the debuffs for the player....
very hard is the perfect gamemode for me, not to easy not to hard