Outside of subjects they are interested in, learning about anything can be quite boring. Even if it's a necessity, it's obvious why a child ends up being distracted by an object essentially designed to be their interest.
The problem with banning these devices in classrooms is that unless you enforce it in any meaningful way, it ends up just being a game of hide and seek with minimal consequences to them (short term at least).
UNESCO calling students to stop using their phones is even less effective than the teachers themselves trying to do it, and we already know how that turns out.