Perhaps it’s just 'cause people don’t think AA guns are cool.
Look at the way people go mad for jet fighters, which (if I remember right) shot down a mere 150 or so allied aircraft. It’s nothing in the scheme of things, but people want to play with cool weapons.
Same with Tiger tanks….overall arguably a minus for the Axis because they cost to much and were so unreliable, but most games seem to include them.
In one way, the AA gun does hit too hard…in reality, fighters and night fighters were much more of a problem. If you don’t have AA, though, it’s too easy to just build lots of bombers knowing they’ll take no losses.
Maybe the answer is to reduce the effectiveness of bombers and AA, but that doesn’t fit A&A well.