It has to do with the mechanics of Aiming Down Sight (ADS) and Aim Assist. Aim Assist is what "helps" you find targets with a weapon while ADSing (you may have noticed the Aim Assist at work if your weapon sways right or left on like a doorway or a wall if there is an enemy within 'reach' of the weapon, regardless if the enemy is in sight or not). No scope will not give you the Aim Assist to lock on-target, therefore it is needed to AIM prior to shooting.
Many people learned how to quickscope, and some even got really good at it. A good quickscoper will put any weapon to shame, regardless of the range.
I still can't get it, no matter how many times I tried. It must be for people with better reflexes than mine, I can hardscope or ADS at a choke point and still miss the target lol.