With heroic effort I can maybe lock down my routers, my kids devices, all devices in my house and they can just go to a friends house and see or, maybe even purchase, every single thing that we[1] have deemed unsafe or inappropriate to children. I am not just talking about porn but drugs, every conceivable type of violence, every conceivable type of crime and criminal, every kind of advertisement, every kind of political propaganda. From the point of view of a parent, the internet is a pipe that potentially connects your child's innocence and naivety to every single bad influence and predator in the world.
Yes, I could keep my kids offline but then they'd pay the price of being complete outsiders.
I don't actually have a solution because I don't trust the government to regulate the internet but from a societal POV of a society that wants to keep existing it's probably best to make the easy path sane and safe per default. That's the way we tried to do it in the past, anyways.
[1] we, as in the majority of people in the majority of societies