Because of their nature. The thing is, mushi like Un aren't structured the same way most forms of life are - their bodies are made up primarily of life force itself, with relatively little actual matter. As a result, most people can't see them clearly, and some can't see them at all.

[The way he talks about this lies somewhere between a rehearsed speech that he's given a million times and a planned presentation that he's genuinely excited about. He loves talking about this stuff, okay.]