Demon Slayer is one of the biggest titles in shonen anime right now, and it's all thanks to a winning combination of excellent animation technique, compelling and heartfelt characters, an exciting combat system and some resonant themes. The series also has some solid humor and comic relief, but sometimes it's more than just jokes.
At first, talking Kasugai crows look like a visual gag, and during the Entertainment District arc, a pair of "muscle mice" were introduced, too. These are intelligent animals that can talk, and their loyalty to demon slayers might be quite telling. There's a reason why demons don't have their own animals pets to help them.
How Crows & Mice Flatter Human Demon Slayers
It's not clear exactly how or why there came to be intelligent mice and crows in the world of Demon Slayer, but the slayers take these animals for granted, and it's evident that the viewers should too. Kasugai crows are the primary messenger system for the demon slayer corps, and ordinary slayers and the mighty Hashira alike use them to receive orders or invitations to meetings.
It's true that in real life, crows are among the smartest non-human animals, while parrots can mimic human speech, and in Demon Slayer, all this is pushed to the next level. Since this is a work of fantasy, not science fiction, the narrative can gloss over the how and why, inviting viewers to conveniently take talking animals at face value. It's practical from a storytelling point of view.
However, these talking animals represent more than the suspension of disbelief. Talking crows and helpful muscle mice are actually flattering to the demon slayers, suggesting that these intelligent animals view the slayers as key allies and trustworthy friends. Not just any animal will allow a person to tame them or put them to work, but Kasugai crows dutifully fly from one slayer to another to relay messages, and Tengen Uzui's muscle mice are proud to aid him.
These animals must view humans as vital allies against the unnatural abominations that are demons, and those crows and mice must also see the purity of each slayer's spirit, drawn to their inner goodness. This is loosely similar to birds and mice being the helpful friends of Disney princesses such as Cinderella, who had mice with her, and Snow White, who had some bird friends. Animals are free of corruption, deception and evil, and they will only associate with similar beings.
The Unnatural Corruption Of Demons
By contrast, demons are never seen controlling wildlife, either flora or fauna, at any point in Demon Slayer. In fact, demons are in disharmony with the natural world, which emphasizes just how unnatural and corrupt they are. To begin with, demons cannot withstand sunlight, the source of most life, and crumble to ashes upon exposure to the sun's rays. This makes them akin to vampires, which are also unnatural, undead creatures.
What's more, demons are vulnerable to wisteria flowers, which are perfectly harmless to most other beings, including humans. To a human, a wisteria tree is something nice to look at, but for a demon, it's nothing more than poison. Clearly, demons are at total odds with nature, including animals.
This may be one major reason why demons are never seen using their own versions of Kasugai crows and muscle mice, with these animals rejecting demons as abominations. These animals also appear to have humanity's best interests at heart, so for more local and conscious reasons, Kasugai crows and muscle mice would never agree to aid Muzan and his minions under any circumstances.
Finally, it's likely that demons are simply too proud to accept help from talking crows and nice, considering such animals beneath them, not even worth eating. Demons tend to be solitary hunters, as Tanjiro noted early in the series, and if they won't even accept help from their own kind, no way would a demon accept help from talking birds and mice like a Disney princess. They think they're too cool for pets.