if ppl build tanky fiora

Not really. She has a lot of true damage with high AD ratio and the tankyish build contains items that actually grant a lot of AD. So would you rather deal 50% more damage or be 3-4 times more tanky? There's your choice.

With a standard heavy ad build (I only play Fiora this way) your passive does around 25-30% a proc. If you get Baron + 5 Dragons then its almost to 40%

People were doing that for the tank build.