you know other people have probably articulated this better than i can, but honestly, on some level, transphobes don’t actually see gender as something that is biological and innate, and they do have a concept of people who are outside of the gender binary - they just don’t see that as a valid identity, but as a punishment. in their eyes, you can fail at being a man or a woman, and you can fail so badly that you are not seen as any gender at all, but you are not allowed to claim this as an actual identity, you are not allowed to intentionally express yourself that way, you are not allowed to enjoy it. gender is something that is given to you and taken from you, and you’re not supposed to have any choice in the matter. your options are supposed to be to conform, or to be dehumanized. they don’t want you to be able to choose anything else.
I think this nails it.
It’s the same way voluntarily childless women are derided. Childlessness is a punishment from God, so anyone who voluntarily chooses not to have children devalues the punishment, which in turn strips the reward of avoiding that punishment. If childlessness isn’t a curse then your five kids aren’t a blessing.
Similarly if someone is happy being anything other than the epitome of their assigned gender then what benefit was there in submitting so completely to that gender themselves?