As it turns out, burying their poop is one of their more explainable behaviors.
Dominance and Submission
Cats, of course, are tidy creatures, but that's not the only reason they cover up their leavings. In the wild, dominant cats do not bury their poop. Leaving their droppings out in the open is a way of signaling that they wish to claim the territory. More submissive cats, on the other hand, do bury their poop as a way of ensuring that the dominant cats in the area don't feel threatened.
Covering Their Tracks
Wild cats also bury their droppings to avoid drawing the attention of predators to themselves and their kittens. The instinct to cover their tracks remains strong in domestic cats. All cat poop smells pretty much the same to humans, but not to cats. They can distinguish between their droppings and those of other cats thanks to pheromones. Because their poop contains unique scents that identify one another, it's important to cats to bury their droppings so that potential predators cannot track them.
To Bury or Not To Bury?
But wait a minute. What if your cat doesn't bury their poop?
There are several reasons domestic cats may choose not to bury their poop, and most of them aren't anything to worry about.
- Your cat may be claiming his territory, like his dominant wild ancestors mentioned above. (This behavior is generally more common among male cats, but not always.)
- It may not come naturally to them to bury their poop. Some cats didn't see their mother model this behavior, and thus never learned. It may not occur to them to bury their poop, and there's nothing wrong with this behavior (other than stinkiness).
- Some cats may find something displeasing about their litter box. Make sure your cat's litter box is big enough that she has enough room to turn around and bury her poop — a small litter box may deter her. Additionally, if she doesn't like the feel of the litter or determines it's too dirty, she may elect not to spend any extra time in there. If you suspect any of these to be true, consider changing to a different brand of litter or a larger litter box.
Now that we've answered that question, we can move on to the bigger issue: Why is cat poop the absolute worst smell in the entire world? We can only hope that science has an answer soon.