Your argument is conflating smart phones with social media apps and you seem to be assuming that kids wouldn't have access to their phone in other locations where they are unsupervised, subject to peer pressure, etc.
The "just say no" argument, basically.
Devices and networks can be configured with parental controls, and the blockage doesn't need to be 100% effective. The kid accessing Facebook from a friend's phone 15 mins a day is tolerable, while giving them access to drugs or a gun 15 mins a day is not.
There is also the education part that for some reason we are ignoring. Kids are going to be able to access drugs in locations where they are unsupervised, they are going to be subject to peer pressure, etc. The job of the parents is to prepare them for that, as they should prepare them for the negative effects of social media.