Why?

So here's the thing.

Everyone says: "Don't buy penetration items on fiora because she deals true damage so she doesn't need it"

"Don't buy crit bc her true damage doesn't crit so it's useless on her"

"don't buy botrk cuz it's useless since she deals max HP % dmg anyway"

But then people buy lethality. But why? She has true dmg already no? I can't understand it. (I can understand hubris tho bc of it's insane AD potential)