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 
Yeah, agreed. I know fuck all about guns but well-implemented iron sights can be a blast, it just feels nice. Same with recoil on a more punchy weapon, or good sounds for the guns. Sure you could strip these "unnecessary" features, and keep the mechanics of how the bullets fly the same, but it is cooler with them
Some games will even give guns "recoil" that's totally fake, just to make them feel more powerful to the player. It doesn't effect the bullets but the visual conveys "stronk!"
Theres a famous example, i can't recall which game, but players complained that one gun was too weak and needed buffs. The devs said "okay, we fixed it" and players declared it the best thing ever. Behind the scenes the only thing that was changed was to give it a punchier, more satisfying sound effect.
It was Wolfenstein Enemy Territory