Broadcast TV had a very simple solution to this problem: Only air the not-for-kids stuff at times of the day when the kids are already asleep, i.e. late in the evening or at night.
It was still the job of the parents to set the bed times etc, but at least this was something the parents could actually control.
And for pay-per-view stations with actual heavily violent or pornographic content: Yes, they were absolutely age-gated, usually via a PIN.
and who sets that pin? It's the parents, not the cable company.
This is correct. I think the difference is that the PIN actually is an effective tool that parents can use to keep their kids from watching this stuff. It's also default-deny as the PIN is pre-set, and the parents would have to make a conscious efforts to allow viewing.
Im contrast, the internet is default-permit: Everyone can access everything, unless the device is specifically set up to block it. Setting up such a block has the risk of causing massive drama with your kids, and they fill probably quickly find ways to circumvent it anyway.
This is why I find the "it's the parent's responsibility" calls so hypocritical: The whole idea behind the internet is to make it as hard as possible to block things. But suddenly we expect the parents to do exactly that? How?
(All that independent of the point that the current push for age verification really seems like a disguised push for control. But that doesn't mean there isn't a real problem. Both things can be true at the same time)