There are too much champs to ban which abuse lethality.
If you ban kha rengar jayce, you get a fucking graves jungle that kills you with qraa every time and you can't do jack shit. Then you get poked by varus q for 800 dmg, or you get ulted by MF for 2k dmg, or quinn flies up to your ass and fucks you and then you turn off the computer and start cutting yourself. Even if you ban all that, someone picks leblanc or katarina and you just want to rip your fucking hair out. Amazing balance boys, amazing.