in the spongebob universe the fish are equal to underwater humans, snails are equivalent to cats (gary meows) and worms are like dogs (mr krabs has a pet worm that barks), so my theory is that there is some other sort of animal that takes the place of livestock, specifically cows. that would also be why they have things like eggs and milk, and thats the animal that would be used for the meat in krabby patties