"Because nerfing them doesn't generally make them less frustrating. You have to rework the [annoying] parts of their kit."
Imagine a world where Akali still had her same shroud and mobility, but her basic attacks and abilities all did 70% reduced damage to champions (dealing one quarter of her current damage would be 75% reduced damage -- 70% reduced damage is slightly more damage than a quarter of what she does now). Would she be frustrating to play against? No, because all the shroud and mobility in the world won't make her annoying to play against if it takes her ten years to kill you. You just disregard her entirely and kill objectives and she can't do anything to stop you if she deals no damage.
Same with Irelia, Kayn, Leblanc, Fizz...the list goes on and on.
Would anyone care about Irelia's mobility, disarm, or damage reduction if she did no damage and couldn't kill you?
Would anyone care about Kayn's phasing through walls, speed, or drain tanking if he couldn't kill you?
Would anyone care about Leblanc's evasiveness and play pattern if she couldn't kill you?
Would anyone care about Fizz's engage range and troll pole if he couldn't kill you?
Or think about this one then?
How many people complain about the slow and knockback of Corki's Package?
How many people care about how much CC Zac has, or how far away he can engage from/how he can gank from absurd places that you can't realistically ward?
How many people are frustrated with how Kalista can save her support from certain death all the time?
How many people hate how Cassiopeia can run you down easily while doing damage without even needing to buy boots?
The answer is, not many. Even if you find yourself in that position because you played against one in a game that was really good, you ban them for a few games until you run into one of the above mentioned popular annoying champions that dumpsters you and then you swap back to banning the popular one, because there's only a Cassio in 2 games out of 1500.
Every champion has annoying elements to their kit. But if the rest of their kit is bad, they still don't see play and nobody bitches about them. Nerf them into oblivion. It'll make them way less annoying even if they still have whatever made them annoying.