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 
I know the lore reason for why guns like the Halo 1 pistol have scopes despite no scope being visible on the model. Actually I think in Halo 5 you could stick different attachments to your guns, one of them being some sort of ACOG dealio you could put on the AR, making it feel more like a gun from COD, which is probably what I was thinking of.
Besides, in older Halos you couldn't even zoom in with non-precision weapons, it was all hipfire or nothing
yea 4 or 5 had a microtransaction situation with unique weapon variants, some of them had scopes that worked like the other in-game virtual scopes