Ehhh, the big factor is that a pickling brine is controlled and small.

You don’t start out with an entire ecosystem of bacteria, fungi, scavengers, and the wide ranging temperatures that exist in an ocean.

Secondary to that, you tend to be dealing with cuts of meat when pickling, not entire bodies.

See, part of what causes decomposition are the enzymes released as individual cells die, and those produced by the bacteria already in the body.

When we slaughter an animal, it doesn’t just get thrown in brine whole. If you did, it would rot from the inside, no matter what the outside brine was like.

Instead, the carcass is drained of blood, organs are removed, and the meat will typically be kept very cool during transport and storage. When you put that into the brine, you’re severely limiting what bacteria are present in the first place. The brine will almost always be made with processed water from a tap, or from a known clean source like wells or springs. So, again, you have a very restricted range of bacteria.

The salt then limits them more. So you’ll lack the bacteria that thrive in salty conditions in the ocean, and only those in the air and fresh water even have a chance to eat the meat before salt kills off the ones that won’t ferment or otherwise preserve foods, including meats.

But! Deep sea conditions are very cold, and there has been footage of scavengers down there eating very well preserved carcasses. Some of that meat may well have pickled to some degree, as some of the fermentation bacteria can handle cold.

So, what it amounts to is that pickling isn’t purely done by the action off salt on the food. Brine pickling is essentially sourdough for meats and veggies. You grow bacteria that prevent the food from going bad in a dangerous way, which leaves you with something that will stay edible much longer. That’s kinda over simplified, but I think it’s good enough for this