Easy
she is the counterplay and everything you do she does better.
Enemy has an assassin? LOL nope. And guess what She's the assassin now
Oh, ok, maybe we can outlast her? LOL nope. Shred, DPS, and a heal.
What about split-pushing and waveclear? Ok, go ahead and try, see how that works.
And last
Ezreal or insert other champion breaks the games when a new item is introduced?
"Oh, you sweet summer child, you should see me with literally any on-hit item."
Not to mention that she is one of few champs that can build basically anything.
Basically, fuck 