They aren't OP, they are just annoying to die to due to the 1 shot mechanic not allowing any reprisal from the opponent. They also take less skill to get a single kill with than a normal gun due to the fact that you don't need to worry about recoil, keeping your aim on them while firing, while taking flinch, etc. All the kill requires is to hit one single bullet, which anyone can do every now and again (especially with the high aim assist and magnetism). Most people don't even check their aim before firing; they just scope in the general vicinity and shoot at the first possible instant, hoping their luck wins out. And if it doesn't, free second chance via tac insert. Its akin to a slot machine that you have slight control over. And I'm sure people are aware of all of this, why else would sniping be so popular? Less skill required, but the same reward (a kill).
That's not to say that going on a streak with one isn't hard. In fact its much harder than a normal gun, which is also part of the appeal for the "hardcore" snipers; they want to handicap themselves and still do as well as someone with a normal gun. The problem with that is the penalty for missing is usually death, and missing will happen quite frequently due to: high player movement speed, erractic/jerky player movement, bad hit detection, etc. The odds of you going on a streak with all of that working against you is very low and again it becomes more about luck than consistent, deliberate skill. Sure your 5 minute montage may make you look like a sniper god, but everyone knows to get that you had to cut 5 hours of you throwing yourself into the feed endlessly, barely able to even get a double most of the time.
To me, snipers are a joke. The gun itself has a legitimately powerful role in certain game modes but most people run around like clowns with it. The only threat they pose is to end your streak early and lower your KDR a little bit more than normal; they'll lose the game mode itself much harder than regular people. I laugh everyday knowing that so many people are wasting so much time practicing a gimmick when they could just be practicing actual game mechanics.