Because they get bored of using the UMP, MP7, and P90. None of the guns are OP since the akimbo FMG9s were patched. Some guns are just better than others, and some people just don't want to realize that.
I personally think any gun you can hip fire with and get damn near insta-kills is OP, I guess I'm just old fashioned and remember the days of COD where you had to..Oh i dont ******* know AIM to get kills
thats what it seems like when somebody just randomly pops around a corner with a rapid fire pp90 doesnt matter if I croutch, go prone, strafe, do a matrix style 360 spinning quad backflip it seems like i always instantly get hit and die