Because is not their place. They aren't doing it to protect their base, or help their friends, they bombing just to bomb. If we we're rolling for the base, and the defenders can't stop us, ok, release the A20's. But there are often GV bombers at an even odds fight without any camping taking place.
If we had a spawnable 90mm with proxy shells, HTC would hear the furballer's screams all the way from texas. The fighters would see it as an encroachment on their territory, and it would royaly piss them off.