Part of the craze of adolescent girls declaring themselves to be boys is a female version of the Peter Pan syndrome of boys not wanting to grow up into men. Puberty is scary for girls and not a few girls would like to just stay the way they are a little longer and not turn into women right now. Lots of parents kind of feel the same way.
Roughly, age 6 to 11 is kind of a halcyon era of childhood that other Great Apes species move through much quicker. It’s a time to learn lots of little skills without being distracted by sex. Lots of girls can’t wait to get to puberty and womanhood and boys, but lots of other girls don’t want to be in such a rush.
So then these natural feelings get plugged into a crazed ideological fad because crazed ideological fads are how we roll these days.