It is true that the more weight of a weapon, the more crazy the damage is. However, the lack of heavy weapons is how to carry them, as well as the limited use of them. For example, it is unlikely that snipers will be used for raid or flooding. AK47 can also sometimes be difficult for users when used in buildings that are of a narrow architectural type. The muzzle of the barrel would be troublesome and slow the user's response when there is a shootout.
When a heavy rifle is unable to overcome the constraints of the place, then the gun will be a mainstay. Yes, this weapon is light, but it has cool functionality. Lightweight will make the user more able to maneuver in spewing bullets. For example more freely move and so on. Hence, the gun is always used as an alternative weapon that must be attached to the member belts.