It's because people have discovered that (1) motte and bailey fallacies [0] and equivocation of language (between, e.g. identity and diagnosis) are highly effective rhetorical tools; (2) merely identifying as something ontologically changes the metaphysical structure of reality [1] which confers certain societal benefits.

There is a matrix of is-diagnosed, is-not-diagnosed, identifies-as, does-not-identify-as, which is open to exploitation by those who "identify as" something they are not diagnosed with. Who gets fucked? The people who are diagnosed but do not identify as their diagnosis.

God help me once we start adding another dimension of people who have a condition, but are also not diagnosed and do not identify with it either...

[0] https://en.wikipedia.org/wiki/Motte-and-bailey_fallacy

[1] https://news.ycombinator.com/item?id=47538165