I'll preface this by saying I'm a novice to dnd, but I am heavily invested. (only been playing on and off for about a year on slow text based tables with a friend group, I watch and read a lot of content) but any time I hear or read something about poison damage it's always coupled with quips of how problematic or inefficient it is, mainly because most monsters are immune to it or resistant.

Okay, so there's that - but there's another thing I noticed;

Player characters don't really have many options to get poison damage. (At least most PCs on average)

Off the top of my head, the only way to get poison damage for any character regardless of class is to buy poison from a shop. There's a poisoner's kit but realistically you need proficiency and work out with your DM how that operates at your table. Aside from that, the only things I can immediately think of is playing a caster class with access to Poison Spray or Ray of Sickness (which I usually hear to be pretty lackluster) or one of the rarer higher level spells that uses it like Contagion.

I know Trickery Domain Clerics get it and I'm inclined to believe other subclasses get it in that same vein but I don't remember any other standouts (Circle of spores druid maybe?)

So my big question is why are so many monsters immune to this when it's one of the rarest/least accessible player damage types in the game? Lol. There aren't even many reliable ways you can inflict the poisoned condition on someone with them - the spells do it for a turn but that doesn't seem like much. Was poison broken in earlier editions that led to this?