Poison is not elemental damage. That is why it is not an option. Elemental damage types are fire, cold, lightning and thunder. There are homebrew feats that enhance poison damage in the tabletop. But it is intended design that poison is resisted by a lot of creatures.
Come on now. The OP said they saw how it works in original rules. If you are going to "well actually" someone at least be fully correct. You didn't list Acid which is an element as per Wizards of the Coast. Prime example being "Absorb Elements" and "Elemental Adept" in 5e both allow Acid.
Now far as "intended" to be resisted by a lot of creatures. First and foremost would LOVE to see a link to a tweet or sage advice or video or anything where either of the creators of D&D 5e saying that "it is intended design that poison is resisted by a lot of creatures."
Resisted and Immune also aren't the same thing and the OP said "thwrated by the number of poison immune mobs". So stating resistances is irrelevant. More over Poison has less creatures resistant to it than most if not all the "Elemental" damage types in 5e. ( Happy to share a link to a resistance / immunity table from straight out of the 5e Monster Manual if you'd like )
It is however the damage type with the most IMMUNE creatures...thus the problem the OP stated. Its a fair complaint as it makes the genuinely fun interactions with poison highly limited post early game. There would be absolutely nothing game breaking about having a feat like Elemental Adept for Poison...especially since the problem is IMMUNITY not RESISTANCE. Honestly it wouldn't break the game to have a feat that bypassed Poison Immunity and treated it as resistance. There is nothing fun or positive about having a damage type available to the players and making it useless mid and late game.
That being said in the end this is an issue with WotC game design and not BG3 or Larian. However....BG3 already homebrews and changes a great number of things so there is no reason they could not have done the same here. They are not beholden to RaW 5e so there is no amount of stating "its how it is in the TTRPG" that will explain away Larian doing or not doing something in BG3. I like the hell out of the game and love Larian as a company but they don't need plot armor or people shielding them. (not saying you are just getting ahead of that response)
