Cisgender: denoting or relating to a person whose gender identity corresponds with the sex registered for them at birth; not transgender.

Philosophy: the study of the fundamental nature of knowledge, reality, and existence, especially when considered as an academic discipline.

If, as a species, we ever come to the point where the phenomena of men identifying as men and women identifying as women becomes a philosophical study, as if reality were something subjective, we will have achieved a level of hubris beyond even biblical proportion.

We might just as well spend our time asking why an acute triangle identifies as an acute triangle and not an obtuse triangle. After all, it's just a label. The conversation would be a waste of every single moment spent. Change the acute label all you want, it will not change the fact that none of the angles are equal to, or more than, ninety degrees. Nothing would change the facts.

If, however, we were to discuss, academically, the phenomenon of non-cisgender, philosophically or psychologically, we might actually come away with some new areas of thought. Likewise, we could discuss the psychological integrity of a person who genuinely believes that an acute triangle can become obtuse simply by demanding that other people refer to the acute triangle as obtuse, but regardless of the academic conclusion, the conclusion will never be anything other than academic.