Because our society teaches them to be social media addicted, super competitive, passive-aggressive, hyper-emotional drama queens that that get all tizzied by the smallest little thing?
Oh, that's definitely part of it.
Part is biology, too. Observing 9 granddaughters' behaviour as they grow up, Mrs. Joe was able to predict their cycles when they got a little older.
One reason I'm glad to be a guy is that my body isn't subject to anywhere near the changes women go through, not just during their lives, but from week to week, in hormonal balance.
When those start ramping up at puberty, some pretty wild emotional effects are not uncommon. Now, overwrite that situation with all you have noted, and the problems of development are compounded, then add in the ability to be smeared, socially shunned, mischaracterized by cadres and covens of patently mean other girls (and there isn't much meaner on the planet, limited only by means at their disposal to be cruel, than a gaggle of teenage girls who have selected a target to torment), and you have an environment I wouldn't wish on anyone.
Small wonder so many of them are getting depressed, and they are often being medicated instead of taught how to cope.