What the charlatans' motives are for this concerted attempt to turn adolescence into evidence of body-dysmorphia I have no idea, though in some cases it seems fairly obvious and thoroughly reprehensible.
I think it's all to do with this paradox:
It’s one of those paradoxes though, isn’t it? Being trans is simultaneously not an illness yet in children the proposed pathways are all medical - stopping natural puberty, giving hormones of the opposite sex, maybe surgery. And those medical pathways are lifelong. And yet the same not-an-illness gender dysphoria in older, middle aged men doesn’t require any medical intervention at all, just feelz.
Many of these middle-aged men start off with cross-dressing, and progress (sometimes) to hormonal treatment and (occasionally) to surgery such as facial feminisation surgery and (very rarely) to genital surgery. The reaction of many people to such men, particularly if they have wives and children which many of them do, is fairly unsympathetic. Transgender children on the other hand, with the 'born in the wrong body' narrative - who wouldn't be sympathetic towards a child with such distress?
The transgender movement needs child recruits. There have always been a few boys who wanted to be girls and an even smaller number of girls who wanted to be boys. But those few aren't enough. Capitalising on the distress and unhappiness that many adolescents experience around puberty was a great opportunity.
This video is a transactivist explaining how this works - "trans youth take the 'sex' out of the trans experience". In other words if you want to legitimise your fetish, make sure it is seen as the same thing as gender dysphoria in children and adolescents, because that isn't about sex. The video should start at the right point, if not, go to 6.54 (unless you really want to watch the whole thing).