I just don't get why it's not an option.

Like, murder was not easy in Subnautica. It was incredibly inefficient, often ended up in you needing to heal way more than otherwise necessary, and for some targets could take a really long time. I remember killing the big Leviathan down in the lava caves because I just didn't want to have to deal with it constantly badgering me all the time while collecting resources, and it took me like 30 minutes of wrangling it with the mech's grappling hook and hitting it with the drill.

I can get the stance of not wanting to give the player conventional weaponry, and pushing you into the role of being prey instead of predator, especially for a game whose main theme is about living in harmony with nature. I have plenty of fond memories of the first game playing with those constraints. but you're just creating less gameplay by making the fish immortal. I'd even say that for someone like me, it makes me less interested in the direction of the franchise as a whole. I've always thought that as far as difficulty curves go, starting as prey and ending as predator is way more satisfying then starting as prey and ending as prey that's harder to kill.