I'm not sure it's a 'choice' any different than becoming a trans woman is a 'choice'.

There's no "choice" in being trans, any more than there's a "choice" in being gay. People who are trans have an internal representation of their gender that is different from what their body presents as. That is a psychological condition - an attribute of their mental state.

Passing isn't recognized as a psychological condition because it isn't a psychological condition. Passing is making a decision to adopt the culture or presentation of a race that doesn't correspond to your internal model of what your race actually is.

It's the difference between dressing in drag and being trans. A person who dresses in drag is making a choice to dress as a woman even though their internal model of gender is to be a man. A person who is trans has an internal psychological self-model of their gender conflicts with their body - there's no choice involved. The person who is passing is doing the equivalent of dressing in drag - they are choosing to "put on" the uniform that doesn't match their internal model. They are choosing to create the inconsistency. The person who is trans is not - they have an inconsistency between their internal model and their morphology that is not the result of their choice.

That's why gender dysphoria and being trans are psychological conditions, and passing or assimilating are not.