Amumu was buffed, the small unnoticeable hit to his base armor is nothing, and he got a new increase in damage for his W.
They aren't touching him because they want to make sure the changes they make work on the junglers like him, if they change him that screws up date a bit. And trying to take away form Lee Sin is harder then people make it seem, he is overloaded, and Riot knows this, the problem is they don't know what to take from him because they want people to still think he is fun. A perfectly legit reason.
Winrate means jack, thousands of other factors are involved with it, you also haven't seemed to actually look for yourself at Lee's winrate, its 45% So by your winrate logic he isn't even op, otherwise the 50 champions above him are.
And get this, Lee sin is widely played by LCS members as people so frequently bring up, he is also brought up on the forums a lot, as I already said, So. Why the hell do you think he gets more attention then Elise? Maybe if people got their head out of their ass for a bit, and decided to be productive, we could make more meangingful threads, actually discussions about taking from his kit without ruining him, discussions about Elise, or the Jungle or whatever.
But since were not doing that we just have to endure the same circle jerk threads about "LEE SIN OP LC$ PLAYS RIOT SUX" every friggin day.