Because quite simply, not everyone WANTS to fire both guns at once. In planes with both MG and cannon, the cannon rounds are fewer and far more precious for quick kills. You dont want to waste them on longer range "spray and pray" tactics. I use my MG's alot of times to try to break a bogie off someone's tail, knowing I'm too far to get a real shot at them. If they see tracers though, alot of times its enough to make them break.
I understand now what you are asking for, but even if HTC thought it was a good idea, I seriously doubt you would see something that requires that much reprogramming while they have so much else on their plate right now.