Why isn't Karthus way more popular in low elo?

Yorick has carried me to silver, but since the new season will kill his jungle role for good, I needed something new. I still play Mundo, I play Sejuani. But let's be real: it's actually fun being the guy dishing out the damage.

So I picked out Karthus. There's an incomparable dopamine rush having finished Rabadons, three points in the ult, and there's a level 13 Jinx farming mid wave at half life and thinking "Don't mind if I do."

Very forgiving if you mess up a fight or two, you still scale to that dreaded global threat. The clear is incredibly fast and you'll get a sizeable lead on most junglers and thus far (sample size ~50 games) I haven't seen a counter that makes the game unplayable.

But nobody contests the pick. Nobody bans him. Why? Is it really that reputation that he's afk farming until R is off cool down? Because while he doesn't have hard cc, his W is quite a useful engagement tool and the damage on his Qs becomes insane. Plus his passive allows one to die and still dish out big. He's dangerous at a scoreline that would take most others out.

And lastly contrary to most sentiments, he's squishy, but quite a strong duelist after the first clear.

What am I missing? I would have never guessed that his popularity would be this low