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.

Even if there isn't an underlying problem, the only real way to change a habit is to replace it with a new one.

I have noticed that usually people who make it their mission to stop doing a thing are replacing that thing with the mission itself. This strategy is always bound for failure, because the moment it starts to work for them is the moment they end the mission. This is when, instead of reevaluating their strategy, they punish themselves for the failure to not do. The cycle repeats, and the person spirals into rumination about their stress.

I didn't just stop biting my nails. I started trimming them instead.

You are correct that working on underlying issues is very important, but there is a huge practical value to blocking out distractions. First of all, it gets you started on scrolling a lot less immediately. Secondly, it makes concentrating on the important parts of life and thus the underlying issues much easier as you have more time and energy now. Thirdly, you will always have some bad days, you will always have some issues in your life that you can't fix (yet), but having a mechanism that stops you from re-entering the habit independently of your current state is really helpful in stabilizing your behavior.

What I'm trying to say is that blocking and working on the "deeper" issues are stronger when used synergistically, it doesn't have to be an either or.

I think this is a helpful reframing, and I have spent time in my life trying to eliminate any possible issues: Improving nutrition, exercise, socialization etc. But my ability stay focused and work on tasks seems essentially random.