You people and your infatuation with balance. Of all the bajillion things that affect pick/win/loss/ban rate, you always pick the most untangible, unprovable one: "because he's inbalanced."
Why won't we just admit that we don't know the criteria for "perfectly balanced"? Give me an example of a perfectly balanced champion and compare him with Lee Sin, please. And good luck with that, because there is not a single champion who is universally agreed upon to be balanced.
What? Warwick? His HP regeneration is one of the best in the game and his late-game scaling is bullshit. He wins 1v1 against Lee Sin at level 3, no joke.
What? Master Yi? You can alpha-strike dodge anything, get free tanking with Meditation, and he wins 1v1 against Lee Sin at level 3, no joke.
What? Amumu? AP Amumu blows the fuck out of everyone and flash+R literally just wins you games. Also, he wins 1v1 against Lee Sin at level 3, no joke.
What? [insert jungler here]? His [insert skill here] does too much damage and he wins 1v1 against Lee Sin at level 3, no joke.
The forums is a considered a joke because Riot, still, after 6 fucking years, refuses to become or elect a final arbiter for gameplay balance. They just listen to whoever's loudest. No one's opinion is considered [expert].