Because it would be unenforceable. What are you going to do? Unlock every child's phone for random inspection to see if they have sideloaded social media apps o

Because it would be unenforceable. What are you going to do? Unlock every child's phone for random inspection to see if they have sideloaded social media apps or not? What if they have a custom launcher that hides the apps? Much easier and far, far more enforceable to say "under 16 + smartphone = seize and fine."