Why is Riot not nerfing the OP junglers?

They are doing exactly that.

The thing is, "OP junglers" at the moment means FREAKING ALL OF THEM.

If your favorite jungle champion seems weak, they are weak in comparison to other junglers, because they can't stand the competition from even more broken junglers. They are still worth about 250% of an average laner.

And no, winrates don't reflect this, they only reflect performance in comparison to other champions in the same role.