>> Phones in schools seem like nothing but a distraction.
> because cellphones became ubiquitous and admins never felt empowered to strictly limit them in schools
You're missing the point.
But there was a time when they weren't ubiquitous and admins did feel empowered to strictly limit similar distracting devices. It's anachronistic thinking to cite the present-day situation as a reason for a past change, when the conditions in the past were different.
This is purely hypothetical, but I could see a process for the change where teachers and admins were ignorant of texting, so allowed phones under the mistaken belief that they would not disrupt class, because students would only use them to make voice calls to parents outside of class (because it would be far easier and cheaper for students to talk to each other in person). Then a new status quo was established. Then the school's awareness of the harms came like a frog boiling in a bot: a little bit a time, and too slowly to motivate action until it was too late.
And the importance of cell phones to everyone’s lives (even children and teenagers) has massively increased since then. For schools to blanket ban them on campus or on the person of students in class would be swimming against a strong tide indeed. My guess is that the reason they don’t is that they believe that to do so would be constant war. And they aren’t paid enough for that.