[not gonna re-post the wall of text as such quote chains have a propensity to grow exponentially]
Dirty Bomb, TF2, and Overwatch being class-based and requiring headshots is an example of when correlation does not imply causation. TF2, Overwatch, and Dirty Bomb, because they are team shooters, all have a significantly longer TTK than tactical shooters like CS:GO. For example, the AK-47 in Dirty Bomb does 14 damage to the body and 28 to the head, whereas the AK-47 in CS:GO does 27 damage to the body and 111 to the head. Overwatch's Pulse Rifle (closest you'll get to an AK) does 10 damage to the body and 20 to the head (adjusting for squishies having 200 hp in Overwatch).
So in CSGO, it takes one or two shots to kill with an AR, and in Overwatch/Dirty Bomb, it takes 4-5 shots (we're assuming headshots here for comparability).
All automatic assault rifles except the scar iirc take 4 shots to kill in close range, with damage dropping off quickly. That sounds a lot more like Dirty Bomb than CSGO to me. This is where the imbalance comes in: in CS:GO, it's fair for sniper rifles to one-shot body shot, because assault rifles can one/two-shot headshot. You sacrifice mobility and economy in favor of reliability in buying an AWP. In Ironsight, even at medium range, it's going to take 6+ shots to kill that sniper--not such a favorable trade-off. Being able to kill with a shot to center mass, having a scope to help aim, and not having to worry about recoil gives him a massive advantage that a CS:GO AWPer doesn't have.
I'm not discussing AVA, CoD, etc. as I have not played those games.