Why do cats bury their poop in their food bowls?
Like other pets or domesticated animals, cats were domesticated from wild animals, so they retain some of their wild survival instincts. Wild felines mostly have the habit of burying their scent, and although cats have been domesticated, they still retain this instinct.
Many people have a narrow understanding of burying feces. In fact, cats will bury their feces in three situations. 
Firstly, it's to bury their scent and cover their excrement to prevent predators from finding them. That's why they bury their poop after urinating. Why don't some cats bury their poop? To put it elegantly, they've been pampered and their natural instincts have atrophied. To put it simply, they're more carefree, or they might even think, "I'm the boss! There are no predators!" Of course, these kinds of cats are relatively rare.
Secondly, it's for food preservation. Wild cats don't have an easy time catching food, and what do they do with leftovers? They don't have safes to lock them away. Large pieces can be dragged up trees to hide, but smaller pieces are simply buried. They'll dig them up again when they're hungry, which is why cats bury their droppings in their food bowls. It's not that they think it's unpalatable; on the contrary, they think it's quite good and want to hide it well so no one can steal it! So, if a cat makes this gesture after eating some food, it means it's full. If it does this right after the food is placed in its bowl, it means it's not hungry.
Some cat owners say: "That's not right! It buries its own cat food when I pour it out, and it eats right away when I open a can of food!" This is actually quite easy to explain. People say women have two stomachs, one for main meals and one for desserts. Cats also have two stomachs, one for cat food and one for canned food and treats. Even when we're full, we still want to try something really delicious, right? Others say: "But when it buries its cat food, it scratches it on the floor!" Yes, there aren't any dry leaves or rotten branches for it to bury, so it just does it as a formality. If you don't believe me, put a plastic bag next to it, and it'll definitely cover its food bowl with it.
Thirdly, they bury unpleasant smells. If a cat finds something in its food bowl that it dislikes, such as the smell of fish (some cats hate that), it will lift its precious little paws and bury it. My cat absolutely hates the smell of banana peels. Every time it sees a banana peel, it spends ages scratching at it. So I thought, "That's great! If it gets too fat, I'll just put a bunch of banana peels in its food bowl every day for it to bury—it'll exhaust you, you little rascal!"
