Like I said before, I can't relate to this. It's not easy for me to answer. If my consciousness ended up in a woman's body for some reason, I wouldn't feel as if I was a man trapped in a woman's body. I'd just identify as a woman. There is literally nothing about my own identity other than my body that I consider to be inherently masculine or feminine.
Not trying to invalidate how other people view themselves, mind you.
The idea that your body is what determines what you identify as is a bit weird to keep bringing up in this thread if I'm being honest. We have countless trans people and nonbinary people throughout history who that does not apply to, and has repeatedly shown that gender identity extends beyond gender roles, gender expression, or even your body.
We can even see this through some studies that have been conducted in the past that show that there is a biological component to this as well :
"A twin study published in the International Journal of Transgenderism found that 33% of identical twin pairs were both trans, compared to only 2.6% of non-identical twins who were raised in the same family at the same time, but were not genetically identical."
(Source:
http://www.hawaii.edu/PCSS/biblio/articles/2010to2014/2013-transsexuality.html)
"Several studies have found a correlation between gender identity and brain structure.
[7] A first-of-its-kind study by Zhou et al. (1995) found that in a region of the brain called the bed nucleus of the stria terminalis (BSTc), a region which is known for sex and anxiety responses (and which is affected by prenatal androgens),
[8] male-to-female trans women had a female-normal BSTc size (like cisgender women) and female-to-male trans men had a male-normal size. While the transsexuals studied had taken hormones, this was accounted for by including non-transsexual male and female controls who, for a variety of medical reasons, had experienced hormone reversal. The controls still had sizes typical for their gender. No relationship to sexual orientation was found"
(Source:
https://en.wikipedia.org/wiki/Causes_of_transsexuality#Brain_structure)
"David Peter Reimer was a Canadian man born physically male but reassigned as a girl and raised female following medical advice and intervention after his penis was accidentally destroyed during a botched circumcision in infancy.
Psychologist John Money oversaw the case and reported the reassignment as successful and as evidence that gender identity is primarily learned. Academic sexologist Milton Diamond later reported that Reimer's realization he was not a girl crystallized between the ages of 9 and 11, and he transitioned to living as a male at age 15. Well known in medical circles for years anonymously as the "John/Joan" case, Reimer later went public with his story to help discourage similar medical practices. He later committed suicide after suffering years of severe depression, financial instability, and a troubled marriage"
(Source:
https://en.wikipedia.org/wiki/David_Reimer)
If that's how you personally feel about your identity, that's fine. But, you asked why people are nonbinary or have a gender identity that does not match their body, and you're kind of fighting against the explanation as it's something that you're not used to thinking about.