You know that children's book, Everyone Poops? Well, they lied. Not everyone does. Sure, real people and animals in everyday life might, but you're not likely to see a fictional character going to the bathroom. This would detract from the plotline, and besides, nobody really wants to see that sort of thing. If something's not crucial to the story, why include it? If the story does mention the subject, it will be for something related to the story, such as a Potty Emergency, or it may be for the sake of Toilet Humor. This is sometimes averted without showing anything by simply having a character excusing themselves for a moment.