Put in other terms, it's like asking why the left-handed percentage of the population rose after left-handedness stopped being beaten out of children.
And the experience of trans people the nation over is not one any person would want to experience. If a person imagined they wanted the adoration and attention of others for being trans, they sure wouldn't want it for long. The mockery from their peers, their elders, their representatives and the literal State actively seeking to "eradicate" them is a *whole lot of harm* just for media clout.
It seem to me there's a normal function of the teenage psyche that leads a small percentage of them to try to fit in as little as possible: goth makeup comes to mind.
Not beating a child is always the right course of action, regardless of whether there is left-handedness that one is trying to "correct".
The kind of "treatment" that someone undergoes for gender dysphoria is incredibly invasive and doesn't seem compatible with the maxim "first, do no harm" unless gender dysphoria were something that had severely negative health outcomes if untreated. Since gender dysphoria is a health issue that's psychological in nature, this needs to be measured against a baseline of psychological health issues actually being quite prevalent even in the general population.
Left-handedness is grounded in physiology and neurology, which are "hard" medical sciences, while gender dysphoria is only grounded in clinical psychology, which just isn't.
When I say "hard" medical science, I roughly mean this: Physiology and clinical psychology both have a history of iatrogenic horror. But for physiology, you need to go much further back in time to find it. And while health intervention on both physiological and psychological grounds have been on the rise, health outcomes on physiological conditions have generally improved through history, while the population seems to be getting sicker and sicker where psychological conditions are concerned.
There are parents that transitions their 4-5 year olds. I doubt the kids would meet any clinical diagnosis or that it would be possible to diagnose at that age.
You can expect a higher percentage that will be diagnosed with dysphoria compared to the former baseline, but I don't think we only see that effect. Perhaps it is even the smaller component.
I also intentionally tried to use leftie stances in sports and I was mindfully doing it just to be different because I thought that was cool when I was like, 7 years old. This stuck a lot more, probably because I learned how to do things with a leftie stance from the get-go.
I think you underestimate how much stupid crap kids will try to imitate if they think it's vaguely cool and how much this can stick in a very real way. If I tried to do some sports as a "righty" I would fall on my ass. I was never actually much for social status either and kids actually gave me shit for not using a right handed stance.