In terms of gameplay mechanics, old school zoom aiming achieves the same thing as ADS. Is getting to press your nose against virtual firearms really that important in a game about shooting werevolves and vampires?
I blame this on the proliferation of Call of Duty over traditional scifi and fantasy shooter franchises. You can fucking ADS on an assault rifle in Halo now 
As far as I can remember, the only literal iron sight ADS in Halo is the Bandit Rifle in Infinite, which is just the Reach DMR without the scope on it. They later added the Bandit Evo which has a red dot optic that works like the BR's sight in H5