Who said that it shouldn't? mk9 AA are actually a reference of how good consistent AA should be to keep jumping beans players in check for the overall health of the game, while still allowing jumping to remain a valid mechanic in moderation. Stop trying to fix what isn't broken, working as intended yadayada.
it shouldn't that's game logic, in fact, hitbox expansion isn't something that existed in MK9, only during mid year of MKX launch forward, but it also seems you don't understand what it is. Its not expanding the hurtbox of the character as he jumps, its having his attacking hitbox overlapped by the hurtbox during the active frames of it something like this:
The red hitbox it's the attacking hitbox, and obviously the blue ones are the vulnerability hitbox or Hurtbox as we call it. inn fact since MKX mid year towards this is what they do, but they do it this way:
While this fixes partially the problem it generates more as the edges of the hitbox and hurtbox expands beyond the character model by a large margin, now you have attacks reaching the hurtbox from a mile away and creating this illusion which ppl outrage on it.
I'm not trying to get aas nerfed, i want a more refined game, if anything jump kicks don't need to stay activated for 7 to 10 frames long that's broken, a jump kick should have a max of 5 active frames and the rest should be recovery.
I also don't remember d3s anti-airing in MK9, though i know for sure they were used to low profile jump kicks not to anti air with it, of course d4 did AA, but not d3.
Anyway, this game has a lot of problems already, just hope they fix the majority of hitbox issues with it.