Just because no one has discovered any definitive physiological reason doesn't mean it's always psychological. Because gender is a construct Some people will choose a gender for psychological reasons, sure, but most of the people that feel so out of place they feel they need surgery likely have something else going on. In the last hundred years ago western society has been obsessed with assuming everything is psychological until proven otherwise, despite there being no good reason to think so, the rationales being backed up by completely speculative unproven and unscientific theories of the 'mind', and despite the fact that our understanding of physiologic mechanisms is still in its infancy.