first, how is their tech? they cannot build AA if they haven't got the tech for it yet, and you can't blame them for that
second, late game production is really high the ai probably can't do it, and they cant effectively rush buy either
Well put it like this: In my most recent game I was 5th in tech. I was playing as English Aquatine and had about 53 techs. Kong way ahead at 64, Australia close behind at 62. Once I got Bombers, it was GAME over for them. They simply couldn;t do anything. They didn't build any anti air defences despite me taking several of their cities, they didn't destroy my airfields, they didn't do a damn thing. Then of course once I took their capitals, there was an emergency called on me. And I was able to simply ride out the turns. Not ONE civ tried to attack me or provide any sort of offense. how can this be right?