Making what might be a bit of a crazy person post because this is my blog and I'm allowed to wear a tinfoil hat here if I want :3
Biological nonhumanity is far more prevalent than we think, and society, whether intentionally or not, is built around hiding and suppressing this fact.
1) Society convinces beings who feel biologically nonhuman that they are deeply delusional and their experiences are the result of psychosis. Even if said being does have a psychotic disorder, I truly do not believe their nonhumanity is a result of this. Sure, there are humans who know they are human, but have delusional episodes where they temporarily think otherwise. But that's not what I'm talking about. I'm referring to beings that consider themselves fully nonhuman, but are told their existence is just the result of mental illness. I do not believe in this. I believe said beings simply are nonhuman, all disorders aside. Being a nonhuman hidden behind a "human illusion" of sorts is bound to fuck all sorts of things up in the body and mind, which is why it may even feel like a disorder to those experiencing it.
2) This view of "biological nonhumanity = mental illness" then leads a lot of nonhuman-identifiers (i.e. therians, otherkin) to be unwilling to explore the possibilities of their own existence. I see so many beings, most of them young, suffering from such terrible dysphoria and so incredibly desperate to have the biology of their 'types, yet they still believe they are trapped in a physically human body because it's not acceptable to believe otherwise. This is absolutely not to say psychological nonhumans don't exists, but rather that the world sees biological nonhumanity as unreal, thus taking it out of consideration for beings who exhibit genuine symptoms of it.
3) While I don't believe that biological nonhumanity, physical transformations, or anything of the sort is something one can choose to have/be/learn, I do think there is a sizable number of nonhuman-identifiers who would be biologically nonhuman if they chose to deny what others see as reality, to deny the "biologically nonhuman = mentally ill" lie, and to instead embrace a reality that feels right to them. All the time I see beings both online and in real life that I feel are just like me when I was younger. My first year after awakening as a therian I spent profusely denying the possibility of biological nonhumanity and truly believing I was physically human. I didn't know any better. But then, on tumblr, I met so many wonderful zoanthropes and shifters, and basically went "shit, you mean I don't have to be human at all?" It was society than held me back. It was the lies pushed by humans, doctors, even prominent figures in the alterhuman community. Sometimes I can't help but look at other beings and mourn what they will never feel like they are allowed to be.
4 (bonus)) I think the suppression of biologically nonhuman beings who appear human could be intentional. I'm not exactly sure how or by who or to what extent, but I think it's possible. I would put more blame on the medical system (specifically the psych system) than governments or private organizations, but who really knows. I think someone out there, some entity, knows beings like me exist, and I think they don't like that, and/or they have some way of monitoring or influencing said beings.