try playing her and seeing how hard it is to actually hit. Most walls aren't that nice on lining it up, and short distance she can't hit because it's only 800 range.
She's basically designed as an Anti Melee.
90% of the time, even if you manage to sleep over a wall, she isn't actually close enough to proc a Q combo (unless it's a thin wall, and the champ is right on the other side of it, not out like 700 range).
I recently had a game where I was vs a Nasus support, Vi, Zed. etc. Basically a melee range team. I will admit. That was an easy game to land stuff on. There was a couple times where I 2 shot the Nasus (who was AP). The Vi was also an easy target, but on average I only took 40-60% of her life, and they surrendered once I had 952 AP.
But usually, games aren't so kind. You can land everything, and still only take 40-50%, while taking 40% chunks to your own hp yourself while an adc like Cait or Vayne pokes you once while in R. If there is a Xerath , he can make existing painful. A Rengar can just hunt you and kill you before you have a chance to do anything. If Supporting, your adc often ignores the sleep and keeps on farming, while you yourself have a huge wall or a minion wave in your way. She just doesn't have a ton of staying power.
In Mid, she can do great vs certain champs. But, she definitely has foes there too.
Even when she is fed, I often barely don't 1 shot..... target still has 200ish HP. And since her combo relies on R to set up the first hit, she often can't get that last 200, unless she has a line of flashes in front of her.
I can more reliably erase 80% of your HP on Evelynn than I can on Zoe.