..why shouldn't they?

Sometimes they don't tell you and just quietly update all of their usernames...

How do you know someone has a PhD?

When it becomes acutely relevant, they'll politely let you know, and then you can become annoyed at them about it.

Thinking about it, that exact thing also applies to other 'how do you know someone is/has/does [...]' as well.

Gene expression is not as straightforward as people think. All sorts of weird shit can happen, and that's not even including gene mutations.

https://static.scientificamerican.com/sciam/assets/File/Pitch_sketch_final.png?w=2000

This is the best resource I've seen to show things relatively simply.

The TL;DR is that a whole "Y" chromosome isn't exactly responsible for "maleness", the SRY gene is. It's normally on the Y chromosome, but mutations can occur placing that gene onto the X chromosome. Inversely, someone could inherit a Y chromosome without that gene, in which case they would develop with female traits.

It's not considered trans because someone with 46XX plus the SRY gene would develop male genitalia, be identified as male at birth, and likely identify themselves as male. For some types of these conditions, there are plenty of people walking around with no clue that their chromosomes don't match their gender.

Disclaimer: I'm not a geneticist, so i could have explained something a little off.

I googled it for you.

https://en.wikipedia.org/wiki/XX_male_syndrome

In 90 percent of these individuals, the syndrome is caused by the Y chromosome's SRY gene, which triggers male reproductive development, being atypically included in the crossing over of genetic information that takes place between the pseudoautosomal regions of the X and Y chromosomes during meiosis in the father.[2][7] When the X with the SRY gene combines with a normal X from the mother during fertilization, the result is an XX genetic male. Less common are SRY-negative individuals, those who are genetically females, which can be caused by a mutation in an autosomal or X chromosomal gene.[2] The masculinization of XX males is variable.

You've heard of xy people and xx people, but wait till you hear about X people!

Or xxx people, or xxy people, or... dies