I think there are probably a multitude of factors, and both social media and too much generic screen time both are unhealthy in quantity. Many things are unhealthy in quantity after all. My oldest is 12, for context. Another factor that I think is really important is that I observe a lot of kids aren't given any responsibility at all. A lot of parents are control freaks. After all, it's easier for me to just do it myself than clean up the mess you make when you do it. 100 years ago you sent your 10 year old out to plow the field because that was the only way the family survived winter. I'm not saying those were better times, but those kids might have had the satisfaction of a job well done, and that's definitely good for mental health.

What's also interesting is that many of the kids that do get responsibility are the ones in really crappy situations. That is, responsibility is forced on them because of circumstances. So these kids might do poorly in school, but end up with a better work ethic than rich kids with education. Or they end up dead or in jail. It's all messed up and there are no easy solutions. We all just need to do better.