Because some people do just randomly shoot with the shotty.
Then it's 1 hit k.o. (most of the time)
Sure you can spray with an smg, but you still have to hit that person numerous times.
I guess you have never used a shotgun. Its not a OHK weapon most of the time. Unless you get some lucky shots at some good range, its a 2hk weapon. Compare that to an ump, which takes around 6. Now look at fire rate between the 2. Odds are, your gonna hit 6 ump shots b4 2 shotgun shots. If not, try a different game bc your aim must be pretty bad.