I don't have kids myself but can refer to my nieces. They have iPhones and access to the internet but they have limited internet time. Not limited in some sort

I don't have kids myself but can refer to my nieces. They have iPhones and access to the internet but they have limited internet time. Not limited in some sort of technological way, but their parents simply tell them they're not allowed to use the devices before 5pm during weekdays and they quite simply listen to their parents. Why don't other parents do this?