Normal AA missiles, in RL and in-game, are not AOE. I mean - yes - they explode - but we're talking about an AOE radius large enough to impact other nearby aircraft - which is a very large difference. There's a good reason that flak style AA weapons are still in use today - along with SAM - and there are excellent reasons why aircraft are generally NOT equipped with flak style weapons for A2A combat, but instead have A2A missiles or simple guns/cannon. (Good thing no one has mentioned enabling 'damage friendly' for flak weapons yet).
Flak weapons are, by definition, considerably simpler, more robust, easier to operate, cheaper - but decidedly less accurate, than SAM - and more well suited to area denial. This makes them ideal for cluster formations (read: gunships) and other low level air threats.
Speaking directly to the original topic: Air units are simply too plentiful and easily replaced - no one thinks about using them carefully.