Gosh I dunno either. The gender binary does exist for the reasons you say Sophie, but as to why some are born (or somehow become) wired to opposite gender stimuli… maybe it’s different for every trans person, with the results being the only common betwixt the lot of us. A brain being born of a different shape, a trauma that reset the stimuli in early childhood…
I think it’s the brain realizing it isn’t getting any positive gender stimuli of one identity (due to any number of situational conditions) and just flip-flops to the other gender at some point.
I know it does this for the eyes. If one is ‘lazy’ and isn’t responding properly to motor control, the brain learns to ignore it and focuses the stimuli on the one good eye.
Additionally if someone puts on special goggles that flip the field of view upside down, and wears them for a week or so nonstop, the brain actually figures it out and rotates its optical signal interpretation.
Maybe the same sort of adaptation can happen from gender stimuli causing confusion or simply not being reinforced in a positive manner. A kid’s growing environment can try too hard to force masculinity/feminine stereotypes, or not be engaging enough at all, and whoops, the kid’s trans via subconscious switcheroo.
The key factor in this hypothesis is that transgenderism is an adaptation to the underlying problem. It’s not the problem itself, which could be severe dissociation, depression, low empathy and so on, but rather it’s the brain’s subconscious solution to it.
Dunno how accurate that is though. It just helps me to think all this since my family’s been treating it as an issue they can’t solve (they’re supportive, they just don’t get it or like talking about it). So, doing research on the ‘why’ question may help you explain to your friends and family, but even having all the answers to where transgenderism comes from won’t explain the euphoric healing it brings.