I think phones may well be responsible. Before smartphones, people regularly experienced true downtime - unstructured moments with nothing to do except be in your surroundings, feel your own boredom, experience your thoughts. Now, the moment our attention isn’t occupied by something external, we reach for our phones to have something to fill the silence.
I believe this has negatively affected everyone’s mental health, but it stands to reason that children would be worse off because they’ve never known anything different: they’ve never had the opportunity to develop the ability to just be.