To be fair.. They "balanced" poisons in patch #1.
Used to be, even though the save DC is low, they're able to be applied multiple times (esp as a dual wielding rogue). But now it makes them immune when it fails, aand these poisons will likely fail the first time, because poisons are con save (DC 13), and npcs are good at con save. When they inevitably fail, you don't get a second try until round 3, because it's a universal immunity.
Compare that to spells, which natively enjoys a scalable, higher save DC, and incurs no penalty/immunity on failure, which means we get to try multiple times on a single round, if we have multiple casters.
Why would you use something that is: 1. cost a bonus action, 2. has lower save DC, 3. incurs penalty on failure, 4. doesn't recover during full rest. ?
It could easily be the case, that before patch, poisons are more efficient and reliable. But they nerfed it (too much), making poisons pretty useless mid to late game.
Or you could look at it as one costs a spell slot or a scroll and the other can be made for free or at least cheaply if you buy the ingredients and one only works on humanoids (guessing) and the other has a broader scope. One only works during combat while the other can be applied at any time and is 'fire and forget'.
