Because killing planes is fun. If they come back sooner, because they died instead of RTB'ing, you have even more planes to kill.
I agree in principle.
But if your base being captured is what your trying to prevent. Then killing bombers after they have already dropped their load is counter productive.
If all your after is a furball. And alot of furballers complain when a base is lost. Then again. shooting down bombers after they have blasted your base to all hell or have tried and missed is counter productive.
It only brings them back again that much sooner.
And if you have killed them after they dropped your hangars and you have gotten shot down or had to land. Then you have no hangars to up from to shoot them or anything else down again.