I dunno man... why does DARIUS get sustain at all? He's a big guy with an axe... why can he magically heal himself? Why can Fiora heal her team? She's a non-magical woman with a rapier!
Ahri literally devours souls in her lore. The sustain makes more sense for her than for most champs that have it. (Like, seriously... what Is the in-universe explanation for lifesteal? just magic, right?) If you're talking about game balance, then I'm gonna have to tell you to suck it up and git gud, my friend... because Ahri is not even CLOSE to the most potent of mid laners.
If you can't manage to handle laning against Ahri because she gets a little heal every so often while she's hurling her spells to waveclear... how do you lane against people like Zed... that can hurl his Q at you from almost a screen away from you every time his W is off cooldown because he's manaless... AND he can leap onto you from the same distance with his ult once he gets it to go for a kill. Oh, and if sustain is your issue... he's AD, so he can very easily rush some lifesteal to do that if he needs.
Now understand me... I don't think zed is OP at all. I think you are just kinda bad if you can't handle Ahri in-lane. She's 75% skillshots. Learn to dodge!