Right, so the answer to your question is because in the past most true damage was a flat amount. So, for example you use Vayne. She has true damage, but it's a conversion for an amount already set to do % health damage of the opponents HP. The amount it does is always based off the enemies health and rank in silver bolts.
Where it started to go wrong was they gave true damage to variable conversion rates, aka Camille Q, Conqueror etc. Those numbers change wildly throughout the game and are part of a champions normal quick rotation. If you can wait a second to get Conqueror/Camille Q you get a big burst of true damage and without the use of an ult.
For another example, take a look at cheapshot rune. Not many people take it. A lot of people think it's pretty bad. It also does true damage, but the amount is a flat number. True damage only gets really crazy when we have abilities and runes converting it into large amounts, it was originally intended as a way to make sure certain characters would be better at getting past tanks and not get stone walled by one on the enemy team. Vayne would be pretty awful without her silver bolts because of her low range, a tank would easily catch up to her and just sit on her until she died. Similarly Jhin has HP damage on his fourth shot to deal with tanks for the same reason, he's immobile and would be easy to focus by a tanky opponent.
I'm probably forgetting something, but the only time in the past we had variable true damage was with Cho'gath ult, and that's meant as an execute.
Vayne has gone through a lot of ups and downs in the recent seasons and they've buffed her several times to keep her relevant.