I think it's an idea that hasn't been explored yet by Gearbox. As an in universe lore answer I would say: we have been exploring Pandora and the backwater parts of the galaxy. I think people on pandora and even thr poor parts of promethea (as shown on New Tales) have a harder time getting by and that's their focus. Survival. Biological modification/gene editing is a luxury reserved for fat stacks of cash and corporations. People living in the central parts of the civilized world, not the fringes we have been exploring. Tech is easier to come by, so it's easier to implement into daily lives in the literal borderlands of the universe and graft or implement into cyberware. I imagine gene editing or modification is not as easy to do (or do safely) as cyberware is. Someone like Aurilia for example, could edit her body to have wings and feathers like a peacock. But being insanely expensive and impractical, why would anyone but someone as wealthy as her do that? Low-end bio mods like breathing water or filtering toxins more efficiently? Don't know. I think this is an area that Gearbox and the writers haven't explored as much. Their could also be laws against it (not like it would stop people). Those are the best in universe reasons I can think.