Actually, the reason is because the people who have those headcanons often delude themselves into thinking it’s canon, and harass people who don’t think so too.
Literally one of the less-than-ten posts on the blog (WHICH IS THE POST YOU STOLE THE IMAGE FROM) debunks the allegation, which doesn’t matter anyway because it doesn’t have anything to do with trans headcanons whatsoever. Ad hominem much?
Oh, sorry, I forgot you shouldn’t expect rationality from someone who literally says shit like this:
“I’m going to cyberbully this bitch so hard”
Truly the words of a cultured logician of the highest calibre, eh. /s