Because it makes one less plane in the air?
If there are 5 enemy fighters over a base and another trying to up and the vulch is there, why let another fighter up? All he is gonna do is ho you because if he goes down he just re-ups meanwhile you have to fly all the way back. I kill any plane I get the chance to. Regardless of if they are landing, taking off, or if there is a con on my 6. If I am gonna die, I am taking as many red icons with me as possible.
Only time I won't kill a plane is if it's been a good 1 on 1 and he is engine out or whatever. By good I mean no hoing and straight ACM.