Several reasons.
They want an army of passable early transitioners to point as "see! If you just put kids on thise pipeline as early as possible they become happy, trans adults." because they know passing is a barrier to entry to being seen as a woman/man.
Another reason is they are peeds who want kids to stay in a prepubescant state as long as possible because they get off to it.
Finally, they project their (very adult) desires onto kids to assume that they know what they want and of course would want to grow up into passable adult troons, despite them being literal children and not truly able to consent to life long medical intervention.
My last reason, and a personal belief, is that sacrificing healthy human bodies to "Science" is satanic and fulfills their sacrifice desires, which is more the case for the parents of these poor kids. Hope the fame and fortune was worth it, mom and dad.