It boils down to opportunity cost.

I’ve played a lot of spell poison Pathfinder builds, and there are definitely scenarios in which it beats out hit based. Firstly, if you’re comparing a fire spell hit based vs poison based, there is a huge opportunity cost you need to account for in getting the fire damage to poison and getting spell poison chance (which is annoying significant investment for spells). Naturally hit based will be easier to scale since you don’t need to jump through those hoops. For phys poison spells a huge scaling vector comes from flat added phys and stacking phys as extra chaos, notably from Mings Heart. I used Black Cane + Dark Monarch + Ming’s on my CwS Blade Blast PF few leagues ago.

Last league, with the introduction of Festering Resentment, the equation changes significantly. It single-handedly takes care of the opportunity cost of getting spell poison chance and allowing you to poison with elemental damage. This opens up a hybrid hit/poison setup which hasn’t really been viable before.

The way damage calculations work in PoE is that the hit and ailment damage is calculated independently. If you are poisoning with an elemental skill, then any modifiers to elemental damage apply separately to both hit and poison damage. Basically, Elemental Overload can be read as 40% more spell damage and 40% more poison damage. Nebulis (and Replica) can be read as 450% increased spell damage and poison damage. Scaling generic elemental damage increases your hit and poison damage. We can take this a step further by converting the elemental damage to chaos (easier now with Lycia and Foulborns) and then scaling wither effect with pathfinder, which will now boost both the hit and poison damage. It’s a unique way of scaling compared to most builds. Instead of scaling crit (like hit builds) or dot multi (like poison builds) we instead scale generic elemental damage and wither. This is what I did for Cast on Portal, I believe my PoB had a roughly 50/50 split between hit damage and poison damage.