They have huge claws that are actually pretty dangerous, and if a predator gets past those, sloths are mostly bone, hair, and sinew, so nutritionally they're not worth the risk.

I love how "I'm just not worth it" keeps showing up as a survival trait. The sunfish is also made entirely of tasteless garbage and isn't worth eating.

That's why we do okay in the water, relatively speaking. Pretty much everything willing to bite us goes "oh fuck no, that's gross" as soon as it gives it a try.

Not really, they're hyper-specialists. Finding food is one of the most dangerous things animals have to do. Predators have to risk life and limb in a fight to the death. Others have to leave the safety of their den and run a gauntlet of those predators for a few hours' worth of food. Cutting down how much energy you need, and therefore how much risk you take, is a common survival strategy, and sloths are masters of it. They're built from the ground up to waste as little energy as possible. For example, you have to contract your muscles to make a fist. Sloths contract their muscles to open their paws, so just holding on to something costs them no energy at all. Most climbing animals hold on to the tree; sloths hang from it, again using no energy. Their fur grows in the opposite direction, so it hangs down when a sloth is upside down. Water just runs off them like a raincoat. Even when they're spotted and hunted, they don't waste energy. They don't panic. They barely feel pain if they're clawed or bitten. They just slowly crawl toward the thinnest branches of the tree, which can't hold their predators. They've survived all this time because they're extremely good at what they do.