Its because the campagain takes a mroe realistic approach on physics.
If you have ever fired or even held a fire arm in your life you will realise that the crap they do in Call of duty is totally not real and completely fake.
If youve ever tried ramming a rifles sights up to your eye it usually ends painfully and its better to lower your sight slowly incase you need to use it immeadiately again if an enemy suprises you.