Narcissistic personality disorder. Same reason they absolutely refuse to be “categorized” by society: because they are better, more special, more unique and interesting, than all you lame plebs confirming to those boring norms.

Here’s an interesting tell: they get mad when you try to use analogy or metaphor to translate their designer identities into a more understandable form. “Oh, so you’re like..” “No! I’m not like that! I’m exactly only this!” They are actively hostile towards any attempt to clarify their gender concepts because being undefinable is the entire point. That’s what makes them “special”.