In the past 10-15 years, teachers have seen a startling shift in children's health overall, but especially mental health. Attention span is out the window. Prying away devices is like pulling a toy from a toddler, often with the accompanying screams and wails (and occasionally violence). Kids have always mocked and gossiped and bullied, but devices and media have turned it beyond 11. Some kids (says my wife and others) seem in a constant state of fight-or-flight, unable to deal with the stress.
Also: it's not just phones; at my wife's school, Chromebooks are prevalent and kids carry them everywhere, often open in the hallways as they walk, using them like phones. Side note: maintaining boundaries with Chromebooks is an even worse nightmare at some schools than smartphones. Google's hard push into K-12 education has been wildly successful; 'Google Classroom' and other tools are adopted by districts and create a hellscape of maintenance, security, and privacy concerns. For both staff and students.