A good question but I think it's because gender dysphoria isn't so much a case of having two separate identities, it's a case of believing your sex is wrong and therefore your alternate sexed identity is the only true one. I think there's currently several different complexes that currently get lumped under the transgender umbrella because ostensibly they result in the same outward symptoms.

For MTF

One is simply gay men who for whatever reason prefer to think of themselves as women, likely due to a lack of self acceptance or an inability to reconcile external societal pressures.

One is a sexual fetish that develops where the man gains sexual pleasure for the thought of themselves as a woman. It eventually progresses to the point where they decide to live their fetish full time. It's a mix of exhibituonism and narcissism.

Those are the main causes, alternatively there may be other causes that relate to trans identification such as wild hormonal imbalances or psychological trauma.

For FTM I think it's initial cause of not accepting that one is gay plays true for a lot of it. But I also think that alternative causes like a good amount of internalized misogynist attitudes contribute to many. The social contagion theory likely plays into that.

There is also body dismorphia which needs to be discussed. I think a lot of women otherwise develop that in response to their breasts growing during puberty and that contributes to trans identification today. I think it's a fairly normal occurrence to suffer from some kind of body dismorphia during puberty for both sexes but it isn't necessarily a sign of transsexual identification unless it persists well into someone's adulthood which is why the social contagion theory should be taken seriously. Time will tell if it is correct if desistance and detransition numbers continue to increase.