Necrotic doesn't have a widely represented ailment associated with it along the same lines as ignite or bleed but it does have dot ailments.
It's a few things but I think the simplest thing is just thematically. Like, just in real life, if someone were to spray a poisonous liquid on your arm, it wouldn't really hurt, it would just feel like water spraying on your arm. Any damage that it did would be from the physical nature of it hitting you.
On top of that, having things be different and unique give them different ways to think about the build. We already get enough feedback that ignite and bleed are too similar. If we let Poison have hit damage, it would be even more similar. The differences are what make these things interesting.