Because I don't believe that kids (or people in generally) magically develop effective techniques for dealing with addiction and bullying when they reach a certain arbitrary age. They don't suddenly know how to deal with complex social situations when they turn 16. They don't know instantly gain knowledge of how to deal with the addictive nature of gamified social media platforms when they turn 16. So hiding them from that stuff, and then letting their first exposure to it be when they're trying to learn to be independent and less likely to look for answers outside of their own experience solves nothing.

Social media can be addictive, screen time can be addictive, and navigating online scammers, bullies and fraudsters is something that takes experience to work recognise and work through.

So I approached it by talking to my kid about recognising what this stuff looks like, about the importance of not revealing personal information, of asking questions if they have doubts. I also let them have as much screen time as they wanted, and told them that the issue isn't how much time they spend in front of the computer, the issue is whether the computer is the only thing they want to spend time with. If they always say no when someone asks if they want to do something else, if the devices start to get in the way of life, and they never want to leave their room, get resentful at having to do chores etc, then the balance is off. And that's what I cared about. Teaching the kid how to fit this stuff in to their life whilst they were young enough to still take my advice onboard.

That's why

Edit - Unrelated, but maybe not unrelated, I also told me kid I don't care if they swear around the house, with their friends or whatever. However, I also told them that lots of people do care if they swear, and they need to be aware of the impact that their choice of words has on the people around them. Swearing at a teacher at school for example, is going to get them in trouble, and "I can swear at home" isn't an excuse for that. I tried to teach my child that the issue isn't the words, but rather, the way your words impact the people around you and your relationships with them.

Kids need to learn by doing and by trying. They don't learn by being told by their parents "this is how it is". And the more hard rules you put in front of them, the more things they have the need to rebel against and push back against when they try and find their independence. And every single kid that has ever lived explores and pushes at boundaries in some form or another. So don't turn the shitty harmful things in to forms of finding independence.

You give kids independence by giving them agency in their own life, and the life experience to use that agency in an informed way.