That's because you aren't a game designer and are only thinking as a player.
I actually wasn't too thrilled with Riot giving Fiora true damage just because they have a bad history with balancing it. But there were some red posts that made a valid argument.
Basically, Fiora as a duelist needs something to combat tanks otherwise she isn't really a duelist and just an extremely terrible assassin due to having a kit with little burst and a focus on sustained damage. Riot could just make her an AD assassin that deletes you off the Rift in 1.5 seconds, but you probably don't want them to do that. In order to ever be worth picking she needs to be able to apply her sustained damage pattern to tanks and very tanky fighters (like juggernauts) or else she will never be worth picking over them.
Riot could give her physical damage...but then we run into a huge problem. In order to ever be a threat to tanks that stack armor and health against her, that physical damage is going to need to be so high that she would turn squishies into a red mist as fast as an assassin...which is the play pattern Riot doesn't want her to have. True damage helps get through resistance stacking, but flat true damage has always been better against squishies and not that great against tanks and bruisers since they also stack health. Percent-based damage is powerful, but if it's physical or magic damage then the numbers need to be insanely high to be any threat to a tank since they have enough resistances to negate a huge amount of that damage.
Enter % Max Health True Damage. It sounds so horrible but does something amazing from a design perspective - It makes it so tanks get hit hard enough to be threatened while squishies aren't instantly blown up.
Here's some math:
Let's say Fiora can hit for 10% max hp true damage.
How many hits does it take to kill a squishy? : 10
How many hits does it take to kill a tank? : also 10
Now lets say Fiora can hit for 200 damage against an armor/health stacking tank and 1,000 against an unarmored squishy.
How many hits does it take to kill a 2,000 health squishy? : 2
How many hits does it take to kill a 4,000 health tank? : 20
Now she explodes carries faster than LeBlanc and takes too long to kill a tank to effectively duel, making her just another assassin that is only good at deleting carries and useless against tanks and bruisers...with a sustained damage kit that also makes her kind of suck against carries.
I personally hate the very idea of a tank!Fiora and will only ever play her full AD glass cannon, but that's just a stupidly risky play-style for most people - especially on a champ designed to not instantly burst her targets - so it's hard to fault them for building an abomination like tanky!Fiora if it's the only way they can survive and if the items are powerful enough to make it work (and good god the items are powerful enough). Her base values and stats are already pretty low and most of her damage is in her scalings as it is, I'm not really sure what you want Riot to do aside from nerf tank items from also dealing damage.
But seriously, if you think tank!Fiora deals lots of damage you better pray you never go up against a good Fiora.