from what i heared ican give you assumptions.

i believe it stems from a deep existential insecurity. if a doctor gives them a diagnosis, that's a great excuse to transition: they dont want to .. it's a medical neccessity.

if a lot of people then 'self diagnose' and are taken as valid by society, that erodes this institutionalized legitimacy. suddenly people think they transitioned, because they wanted to. now all the burden of making decisions falls back to themselves.

on the other hand (or also?), they fear to be ridiculed. they fear 'unfounded' trans people will be a whole in the defense, when the backlash comes.

it's fear, i guess?

edit: i just realised i wrote this was about trans transmedicalist specifically. i think the structure remains in cis transmedicalists as well: trans people choosing to transition means cis people can not identify their gender and their anatomy that easily anymore. it shows them the contingence of their own identity. scary stuff, tbh; but something one can deal with, as proven by the existence of trans people. ;)