Have you been made upset because a certain champion did nearly 80% of your health bar damage to you with one ability? I have.
Correct me if i'm wrong but the reason why a lot of abilities deal % HP Damage is to help prevent the champion with that ability from dealing to much damage to squishies. To be fair plenty of them do deal a lot of damage anyway, but if Fiora did flat true damage she'd be worse against tanks but one shot every squishy she came across.
The question i pose now is why is it that Enchanter's rarely have Heal's or Shields that Heal or Shield based on their target's maximum HP. I don't know about you but i find trying to kill someone with effective 3000 HP just as annoying as being One-Shot from one ability. And i also don't think i'm the only one that finds Enchanter's to be extremely annoying when they end up strong in the meta, but changing their healing/shielding to scale with their target's HP would alleviate that issue and probably make balancing them a lot easier.
Note: They aren't strong right now and haven't really been to overbearing(with a couple of exceptions) since the Ardent Meta a long time ago but it seems to be really hard to balance those champions because it's really hard to balance OTHER champions around the fact that they could end up having WAY more health than they should.