Because it's not just otherkin. It never is.
Whenever there's something fantastic and awe-inspiring, someone's life depends on it. Someone needs it to express themselves, to feel alive, to escape from a terrible world. Whether they feel driven to worship it, fanfic it, fuck it, or be it, as long as they aren't hurting anyone it is fucking oppressive to shame them for it. And that shame is always going to come from more powerful people, and hurt less powerful ones.
That's from the rant we posted earlier. The point is, scratch a prejudice against therians / fictives / otherkin, and you'll find sexism, ageism, ableism, classism, theophobia, transphobia, and/or an alarming lack of disregard for the welfare of both human and nonhuman animals.
The people who believe that they have wrong reflections are all outsiders to some degree. And those people can't be allowed to have their own mythology, or they will forget their place.
If that unsettles you, examine your motivations.