sex and gender operate on a variety of levels.
medical, legal, beaurocratic, interpersonal, domestic, sexual, romantic, experiential, spiritual
it's always easier in a strictly hierarchical world to downgrade your spot in the hierarchy than it is to climb up
born males can easily disbenefit themselves by taking any steps towards feminisation. born females taking steps to masculinisation also usually disbenefit themselves except in certain, contextual scenarios.
so trans women are women = when a born male decides to live as a woman, she casts herself off that throne and down into the pit with the rest of us very easily. i think of course there are complicating factors, but for the most part. loving as a woman, legally being a woman, opens her up to domestic, interpersonal, and structural abuse on the basis of being a woman. whereas for the trans man, the negative effects of being born a female are much harder to elide. legal transition without surgical transition opens up one to situations like being forced into male prisons, shelters, and other spaces while having a vagina and possibly even breasts, which opens one up to higher rates of rape and assault. medicial transition often makes the trans man a domestic outcast from his family of origin or from his cis partner, if applicable, and in the case of lesbians, from his chosen family.
basically, if youre born rich, you can experience the wonders of being poor by just giving away all your money and living on the street. if youre born poor, you cant just identify as being rich.
but trans men can be truly men in their spirit, in the way they love, the domestic roles they take on, and so forth, and might genuinely interact with people as men and get coded as men in social settings. but when it comes to medical issues for his female body, legal issues that affect his reproduction and other rights, and the trauma of having been a woman in a misogynistic world, those are inescapable.