Smartphones give access to information. Lock a kid in a classroom or a detention hall and without that phone they're isolated.

They're also isolated from family and people who might help them if they're being wrongfully detained.

A car is large heavy machinery. Even adults in cars are dangerous. It's a lot harder to cause destruction or accidentally kill someone with a smartphone. And it's a lot easier to make someone disappear when they don't have a working smartphone. Not a close comparison.