In short, transgenderism is a big fad, and the cause is not clear. It is also not clear whether any treatments help.
You may hear that medical societies have endorsed various treatments, but it is all voodoo. There just isn't any good scientific evidence supporting any of those treatments. There are many people who have gotten these treatments and regretted them.
Teenagers suffer a lot of mental health problems, with girls suffering a lot more than boys. Gender dysphoria appears to be just one of those problems, and there is no easy fix.