"Why does everyone who plays Ahri think she's weak?"
Well, why dou you think, that everyone who plays Ahri thinks, that she is weak?
Sure, there will always be somte extremists, who says stuff like that (you can find someone like that for every champion in the game basically), but I don't really see any base for your claim, that everyone thinks that.
Ahri is defintely a strong champ, and I like and play her alot (so, well, I probably would be one of those Ahri-Mains, I guess..). She has a lot of options and possibilites in her kit, since she is kind of a jack of all trades. So whatever happens in a game, and even wehn behind you will always be able to stay relevant and to adjust to the situation. Plus, you can play her really safe, thanks to your ultimate. Which is, I think, the biggest reason for her high-winrate. Not, because she is so strong, that you win games with her so much. But because you can play her, so you don't lose the game for your team. With other champions, you either win the game oder you became not only irrelevant, but you also become easy targets for the opponent. Thay aim for you, get even more kills for their team and continue to win the game.
With Ahri, you have alway the possibilty, to use your Ult defensively. Sure, you will lose the offensive-kill pressure. But when you are behind, you don't really have that anyway anymore. So, you play it safe, deny the enemy to snowball the game, and find a way back into the game by altering your playstyle to the more defensive/kiting way. I don't really think alot champions are ably to change the way of playing them that flexible as Ahri.
Atleast thats what I think about the reasons for her high-winrate.