The majority of the player base hate Yasuo and Lee sin

Majority of this board =/= Majority of player base*[slayer-pantheon-popcorn]*

Lee Sin as he currently is is the poster child of League of Legends, funny to look at, relativley easy to get into, satisfying when you're doing good with him. He cashes in for Riot in more than one way.

Additionally, both champions have a large, dedicated playerbase, that is likely to buy skins for them, and if Riot fucks up the potential reworks like they did with let's say, Akali, Sejuani and LeBlanc, people will drop them, leaving their former posterchilds in the dumbster, seeing no play and being irrelevant.

Tl;dr: Riot makes bank with 'em.

Majority of the player base = Low ELO.

I don't think I need to explain why you shouldn't balance around low ELO.

Why should they? First of all, Boards is not entire playerbase. It doesn't even make up 1% of it. Second of all, no one gives a shit if you or others hate them. That's irrelevant. Third of all, they are balanced whether or not you like it. Fourth, they do just fine with their current kits.

Because if we reworked every champion that the boards has ever hated, this game would be generic and dead by now.

Because they're riot's favorites.

Because if Riot changed/removed the parts of those champions that people found frustrating, they would no longer be those champions. Instead, people should learn how to fight them properly. It's quite clear (and it has been for a while) that Lee and Yasuo are not OP, so the only thing stopping people from succeeding against them at a reasonable rate is themselves.