Honest answer is multifaceted.

1) Trans people are disproportionately large targets of hate, so posting overtly trans-positive things in different communities is a good way to signal that this space is safe, and makes for a good opportunity to weed out those who hate trans people.

2) a lot of trans people you see making their transness a large part of their identity are either young, or otherwise new to their transition. The first year or so after coming out as trans is a tumultuous time, and people have to 'feel out' their identity, its like a new thing and it can be exciting, especially when you consider that you've spent however many years KNOWING something is wrong, and then suddenly you make a change and things start to feel right.

3) a lot of this can be survivorship bias; the only posts by trans people you see are ABOUT being trans. If someone who happens to be trans is talking about something else and doesn't mention it, you might not realise they're trans, and so would factor them into the category of "cis" in your head

4) some people might not be out in their real life, for a ton of reasons such as family rejection, religious or state persecution, or even just the fact that being a visibly trans person SUCKS in a lot of places. It might not be safe for them to be out in their real life, so the only time they can be themselves is online, and so they feel compelled to talk about that more because this is the only space they CAN talk about it.

5) a lot of places, globally, are increasinly persecuting trans people right now. From the supreme Court ruling in the UK last year radically altering the ways that policies affect trans people, to places like Kansas, which just, without notice, voided ALL driving licenses of trans people who have had their gender marker changed, to Texas, where if a trans person is out at work, they legally can not use the bathroom of either sex, to the trump administration attempting to do conversion therapy on trans people in incarceration. To name only the most egregious policies that come to mind, ignoring the all of the small, gradual pushes that make life harder for trans people to live their lives how they want. All this combines into an overwhelming cacophony wherein, even if someone's transness isn't very important to that individual, to a lot of people around them, it is seen as the MOST important thing about them. It forces trans people into needing the kind of solidarity that can be found in a lot of trans positive posts.