Yeah, I know LMGs are heavier than ARs and SMGs but its a game, we dont need pure realism. Strafing with LMGs is really slow and that type of gun is inferior to SMGs/ARs.
Thats like to say in other RPG´s "why is the knight not as swift as the thief?" ....
If we wanted realism - people would trip and fall trying to strafe. There you go. You aren't realism but some semblance of one.