Not OP, but serious answer to a bullshit question:
Because being trans is not a mental illness. Dysphoria is a medical condition, and transition is its treatment. And it is incredibly effective treatment. When able to transition young, with access to appropriate transition-related medical care, and when spared abuse and discrimination, trans people are as psychologically healthy as the general public.
Transition is also the only effective treatment for dysphoria. Attempts to treat trans people’s genders as a “mental illness” to be “cured”, aka “conversion therapy”, was the default medical response to trans people for decades, and it never worked. Drugs, talk therapy, classical conditioning, electroshock, every method imaginable was tried, and they produced nothing but a wake of ruined lives and suicides. Which is why these attempts are now condemned as worthless, pseudo-scientific abuse by every major US and world medical authority.
Meanwhile, transition works. It vastly improves the mental health, social functionality, and quality of life of those who need it, while drastically reducing rates of suicide attempts from about 40% prior to transition, down to the national average after transition. Again, when able to transition and spared abuse, trans people are as psychologically healthy as the general public.
Which is why transition is recognized as medically necessary, frequently life saving medical care by every major US and world medical authority.
Citations to follow in a second post.