For whatever reason the support groups I went to early in my transition had more than a few transmedicalists. I didn't know any better at the time so I'd hear them out and discuss things with them. Purely anecdotally I can say that there are a few different reasons they hold the beliefs they do.

The most prominent reason I encountered was confirmation bias. Medically transitioning for them was necessary and so it must be necessary for everyone else too. The people I met could not understand how someone could identify as trans and not wish to transition.

Another group was driven by internalized transphobia. To them the idea of a trans person not medically transitioning and not caring about passing was abhorrent. They would use bigoted phrases like "man in a dress" ("brick" hadn't hit the scene yet or they would have used that too) to describe people who weren't medically transitioning and it always seemed like they viewed them as lesser, if they'd allow them to claim the trans label at all.

Finally there were some people who sort of jumbled those up and tried to put a veneer of legitimacy to their bullshit by claiming it was a purely political stance. They were of the opinion that the trans rights movement needed an ideal trans person and that individual was at a hetero-normative end of the gender spectrum and had complete or mostly complete passing privileged. They would even get uncomfortable and shifty about gay/lesbian binary trans people. Any deviation from that ideal, to them, muddied the waters and lead to alienating the cis people who are de facto in charge of what rights we do or do not have.