So what about Fiora?
Most tanky champions have a lot of power in their kit that isn't reliant on damage itemization.
Tanks bring CC.
Most bruisers have such a hybridization of physical/magical/auto-attack/ability damage with the occasional %HP, %armor pen, and/or true damage they'll still deal meaningful damage without itemizing specifically for damage. Usually the base damage is pretty ridiculous on such champions as well, Nasus can get a potential 300 AD steroid from his ultimate, Mundo's AA steroid is 100 to 200.
Anyways - a squish can't counter-build any of these with a targeted defensive item like Thornmail or Banshee's Veil. Neither can a lane opponent.
Some tanks and bruisers also scale damage with defensive stats, so building that armor/health/MR on champions like Shen, Olaf, Volibear, Rammus, Malphite, and a few others will also increase the damage.
Fiora is one of the select few champions in LoL with no CC at all. She also brings no utility to her allies in any form. All she does is damage - and it's almost entirely physical auto-attack damage, with a kit that's easier to counter-build than that of a decent number of safer champions with more range like Kog'Maw, Ryze, and Lucian.
Basically - Fiora has no presence with defensive itemization. And pretty much every champion is going to win in a tank-off vs. her, for reasons listed above. Only time to run into defensive adventures with her is against pretty awful team-compositions (which can happen reasonably often in Dominion/Treelines/ARAM).
Flip side of damage being easy to counter-build is Fiora scales harder with things like AD, armor penetration, and critical strikes than a solid 99.99% of the roster - as all those multipliers hit basically her entire kit's damage output in one go.
Most top-lane champions have such spread out damage they're better off ignoring AD and AP entirely and get their damage in things like Wit's End, Sunfire Cape, Trinity Force, Iceborne Gauntlet, Blade of the Ruined King, Feral Flare, or other such items focusing more on cheap-early, no late-game damage scaling things.
Where a tanky top-lane champion will often see better results with tanky itemization to bring presence, Fiora brings much more with a lot of offense.