I know there are mods but I'm speaking vanilla. Seems like such an obvious thing to have. I assume it has something to do with balancing? Since breathing affects consciousness and rest rate, boosting lung efficiency by two artificial parts would improve pawns performance too much? Seems unlikely, numbers could be easily toned down and there are no archotech OP internal organ bionics. I would like to have those artificial organs in vanilla game, both to more easily replace shot organs without the need for harvesting (especially nasty for ideologies that abhor such practices) or relying on unlikely trader RNG and hopefully for better durability.
Or why not a fully bionic replacement body ?
My guess is that the natural organs you see transplanted in real life are meant to have the moral heavy choices in Rimworld. Having all of them as bionics available makes the choice a bit obsolete. It's rather easy to make bionics at will at one point in the game, but someone ruining their organs due to drugs still needs organ harvesting, a rare buy or live with the consequences.
The heart is a special case because there is a bionic version of it, but seeing the metaphorical gravity of a bionic heart as one big step into something beyond human, this seems acceptable.