Imagine being a teenager and trying to include someone lacking a cellphone to your social group. It adds a huge amount of friction.
Including someone without a cellphone means you give up the ability to be spontaneous (assuming the kid needs to adhere to an agreed-upon schedule to be picked up by a parent), and trivial things become a literal telephone-game ordeal.
It also means almost invariably that the kid is relying on a friend's phone to communicate parents / a ride, unless that kid is so privileged as to own their own car.
Could you imagine if every time you hung out with a friend, they asked to borrow your phone, open Facebook Messenger (or whatever sensitive messaging app you use), and to contact a parent?
It also means the kid isn't included in group chats, which means they just aren't part of conversations other kids are having. You can't have a rich social life with someone you can only talk to in classes you share and for 20 minutes during lunch break.
The "green bubble" phenomenon is bad enough as it is. I don't like it, but for all practical reasons, owning a cellphone is necessary for a kids social life in the United States.