Because you don't engage manually any more. As I pointed out in above, the A (air superiority) version has only 120 rounds of ammo, which is three seconds worth.
If you're using the gun you tell the computer "I want to shoot that guy", at which point you try to get him onto the pip on your display, which is going to move around depending on your relative velocity and distance. The computer actually fires the gun when it decides the round will hit. If you had to do it manually you'd most likely run out of ammo before you hit anything.
The air-to-ground system is similar.
So the software isn't just firing the gun. It's integrating data from all its sensors to figure out when to fire based on what you're trying to hit. I assume they have cut-outs friendlies as well, so you don't accidentally shoot your wingman.
If you're using the gun you tell the computer "I want to shoot that guy", at which point you try to get him onto the pip on your display, which is going to move around depending on your relative velocity and distance. The computer actually fires the gun when it decides the round will hit. If you had to do it manually you'd most likely run out of ammo before you hit anything.
The air-to-ground system is similar.
So the software isn't just firing the gun. It's integrating data from all its sensors to figure out when to fire based on what you're trying to hit. I assume they have cut-outs friendlies as well, so you don't accidentally shoot your wingman.