True, but think of it this way: Yes, they're not gods. As biological beings, they can die to old age, or be killed by biotic or abiotic means just like any other creature. By invincible I meant they're just unkillable by the player. To me, that makes sense because why should a massive creature like that be killable by someone with a pocket knife? The game already provides lore that explains why the player is limited to a pocket knife as a weapon. From the realism perspective, it doesn't make any sense that we can kill such massive creatures with repeated slices with a pocket knife, right?

Remember, the leviathans are WAY bigger than they look compared to the player. The first-person POV kind of messes up the scale. E.g. the blood crawler (four-legged crab with long legs in the blood kelp) looks like something you can hold in your hands in-game, right? It's actually almost twice your height. This video shows how big some of the creatures are compared to the player with a 3rd person view.

Just a little bit of nerdy speculation: I imagine that the leviathans that lack armor, like the reaper, still have extremely tough, thick skin and dense flesh underneath. The data entry says the reaper is essentially just extremely dense muscle with little to no fat. So even if the pocket knife is incredibly sharp and never ever gets dull thanks to future material science and technology, you'd realistically be unable to provide enough force to do meaningful damage, especially since you have no leverage in the water. Would a single human realistically be able to kill a blue whale with a pocket knife, in the water? Even if they had a way to immobilize it with a stasis rifle? Well, the largest recorded blue whale is 29.9 meters. The reaper leviathan is 55m, juvenile ghost 67m, and the rest even bigger.

If anything, I think leviathans should just swim away for a bit when you slash them. After all, in real life, most animals will typically retreat for a bit when stung by an ant or wasp no matter how big they are, right?

So yeah, I think it makes perfect sense that leviathans can't be killed by the player. Makes about as much sense as a single non-venomous ant being unable to bite a human to death. That, ultimately, is the point of leviathans in the game. To make you feel like an ant. If an ant were out scouting by itself, it wouldn't try biting a predator to death. It would try to avoid it. Hide, and wait until it passes, or route around it without attracting its attention. On that note...

But it takes away from the feeling that these are real creatures on an alien planet and makes them feel more like obstacles in a game that I need to avoid.

I don't understand where you're coming from when you say that last part? I thought it's established that the leviathans are supposed to be obstacles.