Because they are threatened by possibility of reproduction without men. The idea that trans women are not actually women or that their womanhood is dependent upon their sterility is a way to deny the fact that two women without any outside assistance may reproduce. The idea of autogynephilia is another way to deny the reproductive capacity of women. Reproduction without patriarchal domination scares them. They are terrified by women who without need of donor sperm, can create a child. You’ll note how male pregnancy is regarded as less threatening than impregnation by a woman. I’ve noticed that in media, trans women are only allowed to be bio-mothers if they got someone pregnant before they transitioned and are no longer coupled to the person who bore their children (OITNB, Hit or Miss) .

Hell even the fact that compulsory heterosexuality is stronger for trans women makes sense given this logic.

The transsexual empire is the opposite of what’s going on, gate keeping is patriarchy reluctantly trying to regulate a transgressive womanhood it wishes would just go away. It hopes by forcing trans women to conform to heteropatriarchal standards of womanhood, by sterilizing them, by denying the legitimacy of their womanhood if they refuse to comply with these dictates and making it at best conditional when they do, to cut off the possibility of resistance they represent
www.advocate.com/politics/transgender/2015/02/23/watch-most-trans-europeans-face-nightmare-forced-sterilization-divor

m.motherjones.com/mojo/2012/02/most-european-countries-force-sterilization-transgender-people-map

This might be a Foucault-y baseless assumption but IDK I feel like I’m onto something