RatRolyPoly wrote - "I'm definitely not making a direct connection between transgenderism and homosexuality (yes, yes, one is a sexual orientation), but I am genuinely curious as to why our inability to currently identify transgenderism in the brain means it can't be something present from birth."
Today, heterosexual people everywhere are sexually attracted to people of the opposite sex. Homosexual people everywhere are sexually attracted to people of the same sex. It was the same 1,000 years ago and it will be the same 1,000 years in the future.
In brain difference studies, sexuality is a relatively static target in terms of the behaviour of people in both time and place. It still seems to be proving very difficult to find brain differences, even after decades of searching.
A male child who likes kicking a ball is currently gender conforming in Brazil, the UK or Germany. If he spends time in other less-football-centric societies, e.g. the USA, he is at risk of being assessed there as gender non-conforming, because different gender stereotypes prevail in those societies. Nowadays, he is at risk of his behaviour in less-football-centric societies being used to diagnose him as transgender, at least for the duration of his visit. If he then returns to his home country, he is suddenly gender conforming again. At home, is he still transgender or is he not transgender again, or is he something else? How will scientists control their experiments to cover such scenarios? Supporters of gender identity expect brain studies to explain this gender conformity and non-conformity as "nature" not "nurture".
In brain difference studies, gender identity is a moving target because it varies with time and place. I think gender identity brain studies have to control for more variables than are involved in brain studies of sexuality. I'm not impressed with the quality of gender identity experimental controls that I've seen so far. I don't expect gender identity to be found in the brain in the near or medium future, but I won't be surprised if some people claim this prematurely.