It's a mix between REALLY bad weapon balance and high RoF guns bypassing server tick issues since they're vomiting out a clip in the time it takes for one of these 60hz or 45hz ticks to go off.
This is why SMGs are massively over performing while LMGs suffer so much. SMGs clip size and accuracy mean they can dump a mag in a second and many of those hits are going to register.
On the inverse you have LMGs dumping gigantic mags and even when you see hit detect work (blood on shots connecting) half the time the server doesn't update and those shots don't deal damage.
The tick rate/hitreg is a severe problem...but even if we ignore that...SMGs just deal too much damage and have too little bloom and/or recoil over large distances. The only times they're ineffective is when your scope can't make any details out to actually hit something.
It doesn't matter if SMG damage falloff is massive at range when you can accurately magdump into someone's head and do it so fast that the hitreg problems play in your favor.