There is much symbolism behind many of the images relating to these ideas. At least, that’s what Abeno thinks, having been around demons his whole life, despite being human himself. With only a select few humans able to even communicate with demons, bridging the gap between the two seems nearly impossible they are too different to connect with each other.
In most cases, demons look closer to an animal or plant (excluding the more powerful ones), and their animations have a 3D, CGI-esque style that normal, human animation doesn’t have. Beyond that, the demons are very distinct in appearance, which only seems to widen the gap between both races. Then you have the humans: neither seeing nor believing in the demons, who often (regardless of intention) cause pain through their ignorance. On one side you have the demons: an invisible race existing alongside yet not being noticed by humans. Not only that, but the show likes to highlight the stark differences between human and demon culture by putting both side by side using novice Ashiya to ask questions and react like a normal human would.