One thing I noticed is that when I changed the settings, the computer did purchase aircraft for its side, but never used them. I wonder if it is buying the planes but setting an airstrike.
I still don't understand why the opposition never has helicopters too........
I don't know, but it might be something you are doing that is discouraging the enemy from using their airplanes. Do you usually buy tons of air defence that scares the enemy away? Or do you just move too fast for the AI to call down fire on you?
I've continued playing around with the settings, and now see that the opponent does purchase aircraft and uses them around turn 4 or so. So one mystery solved!!!