Based on my interactions with teachers, the administrative class that runs these schools are cowards who don’t want to deal with angry parents, nor the liability if the phones get confiscated and then stolen/damaged. There’s also a lot of parents who want to text their kids during the school day and get mad when they can’t. A lot of teachers have given up since the higher ups won’t back them up. This happened around 2015 or so, when smartphones became ubiquitous.
I’m currently rebuilding my math foundation and part of that process was tracking down high quality educational resources with passionate instructors, rigor, and entertainment factor (because I want stuff to recommend to parents). I did eventually find something that was better than what I got in grade school, but I have to say that the Pythagorean Theorem just isn’t going to be as interesting as social media feeds and entertainment products custom tailored to my preferences. No teacher is realistically going to be able to compete with the multi-billion dollar entertainment industry for attention and tech companies are abusing psychology research to make their shit as addictive as possible. It’s not the biggest problem with the US educational system, but it is one of many, so I’m down with restricting smartphone access at schools.