This is what i mean by factions
This goes into a breakdown of who is immune to what, like all fiends are immune to poison, as are all elementals, and undead iirc, those are enemy factions
Outside of certain factions, almost nothing is resistant or immune to poison
No beasts aberrations monstrosities or fey, none or barely any have defense against poison, where as some factions are entirely immune, conversely, say, acid is resisted by one faction (slaads, although they might be a family of creatures in the faction of fiends, don’t remember) and a very small handful of outliers are immune, necrotic gets a lot of resistance/immunities in the undead and celestial factions, but it’s not all immunities, its a lot of resistances
The thing of why poison is hated is that it’s all or nothing, X creature probably seems like it’s got no resistances, or it’s immune to poison, and yeah, a lot of beasts and whatnot get generic resistances at higher CR and don’t really get poison, it’s the energy, seeing poison damage is a feelsbad