It just isn't something that fits into the game. You're also forgetting that ACU already has the protection of an engineering suite.
T3 ACU can build itself a t2 shield and a sam in <15 seconds.
If you die to gunships with your ACU poorly positioned you deserve to lose. You are supposed to lose. Not to mention scouting and the variety of other ways to avoid being air sniped. We're not trying to make it more difficult to lose. Those are the simple weed-outs between <1.5k players and >1.5k players that create a high skill cap game.
This game is a series of investments, and what you're proposing devalues T2 air investment (and jesters) and adds precisely nothing in return. Air doesn't need any sort of nerf, it is already incredibly expensive.