This would not be level-scaling on the character's level but would be rather based on the mob's level. And this would not even be a new unprecedented system in D:OS - I mean right now the HP of mobs and their physical damage output is also based on their level (and their weapon), isn't it? Generally speaking: higher level mobs -> higher damage output. So, why shouldn't this system be also extended to poison?
My guess - which I have said before a few times now - is that it's because Poisoned, Burning, and Bleeding status effects also receive level scaling like skills and spells do, and like how health increases as your level increases.
