I am not a doctor, but I am a parent. I think the problem is that kids cannot learn to cope with their problems and anxieties when they have zero idle time. Kids need time in the day to face and mentally process the social and hormonal challenges that they experience in everyday life. They simply cannot do this when all of their "boring" down-time is consumed with the distraction of devices. Youtube, chat, news, games, you name it.
Kids fill their idle time with all of this interactivity which keeps them entirely distracted from life's challenges, blocking them from achieving critical mental skills.
I also think this is why we are seeing the rise of so many "fidget" toys. Take away the phones and kids are desperate for some other form of distraction. Sure, some kids are natural fidgeters. But most are simply looking for another distraction from life's challenges.