Generally, I think it's boredom. Specifically, many people haven't learned it is a life skill to deal with boredom, instead they constantly find a way be entertained and a phone always within reach offers a great solution to that problem. I say, force yourself to be bored without a phone. Pick up a book, pick up a hobby, go outside, talk to people (meat space), gardening, exercise, etc.

Even people older, like me, who grew up without these things for a good portion of their life. They lost the ability to be bored and need to relearn it.

I personally have always refused to get sucked into the phone. Never turned notifications on, never cared about social media, etc. I don't like video'ing the concert I'm attending. I like being present and I love being bored.