Because if the clips are anything like the ones I've seen, if the gunner wasn't dead, there wouldn't be a clip.
I dunno what clips you have seen but the average LW pilot had only 2 % hit rate against a bomber firing from 300 yards. How close do you think he needs to be to aim at and 'kill' a gunner'?
There's plenty of LW accounts of shooting bombers at point blank range with very few rounds. I quoted this above:
...like the armored knights in the Middle Ages, we were well protected. A Staffel might lose one or two aircraft during the advance, but the rest continued relentlessly on. Positioned now about 100m behind the bombers the Staffel leader barked out the order to open fire:
Pauke! Pauke!...
From such a range the Staffel could hardly miss, and the 3cm explosive rounds struck home. Just 2 rounds could take the tail off a B-17. A B-24's fuselage structure was not as sturdy. The enemy bombers literally fell apart in front of the Sturmgruppe.
They had to get in close just to hit the bomber, even closer to hit specific points on the bomber. According to the above pilot he says it took two 3cm hits to take a tail off a B-17, how many does it take in AH? AH bombers are neither to fragile nor were the gunners accurate enough to fend off fighters.
As MrRiplEy[H] said bombers were cannon fodder left alone with out escort.
Bombers in AH are far more lethal and accurate then they were in RL. This a game play decision not 'realism'.
If the bomber folks were forced to fly bombers as they were in rl no one would fly them. They are boring enough as is but would be even more so if the were forced to form up in a large section, climb, take an in direct course to to the IP. Not to mention who's is going to escort them on a long boring flight?