AI Gunners do not fire AT missiles at ground targets.
Gunships do "attack runs" but to not fire on their target.
The AI gunners are not even looking at the target, but somewhere off to the side.
Also to mention: Tank Gunners aim at air units, but they do not engage with their MG (7.62mm) or main gun. (Maybe a similar problem?)
7.62mm is effective against most helicopters, so why not allow AI to shoot at them?