True damage is getting put on champs/moves that aren't supposed to care how tanky you are. Vayne is supposed to shred anyone that gets close to her, Garen is supposed to be able to enact justice upon the wicked no matter how tough they are, and Fiora is supposed to be The Duelist. It all sort of makes sense thematically, but most importantly %max health true damage is a way to make a champion feel powerful without making them overbearing on squishy targets. If they want Fiora chunking tanks for 20% of their health when she hits them, then you can be damn sure they will make sure she does. If that means giving her 40% physical damage so that it does 20% when the tanks armor negation and your armor pen play out then that's what they will do, which then makes the move OP on squishies because they have no armor.
Basically the point is that even with all the items you have shown there a tanky target will still have more health and more armor than a marksmen (60% armor pen/reduc on a 200 armor target > marksmen armor at lvl 18) , requiring the move to deal more physical damage in order to have the same effect, making it obscenely powerful on weak targets in order to do intended damage to tanky ones.
TL;DR : Even with a full tank shred build a tank will survive more punishment than an adc, making %max health true damage the simplest way to make moves feel powerful on everyone without dealing obscene damage to squishier targets.