Because there are network operators who have mal-intent increasingly no network operators are permitted to exercise network-level control. A parent who wants to filter the network access in their house is the same as a despotic regime practicing surveillance and censorship on their citizens.

Given that it's pretty much the norm that consumer embedded devices don't respect the owner's wishes network level filtering is the best thing a device owner can do on their own network.

It's a mess.

I'd like to see consumer regulation to force manufacturers to allow owners complete control over their devices. Then we could have client side filtering on the devices we own.

I can't imagine that will happen. I suspect what we'll see, instead, is regulation that further removes owner control of their devices in favor of baking ideas like age or identity verification directly into embedded devices.

Then they'll come for the unrestricted general purpose computers.

If you have a device you don't trust, don't allow it on your network, or have an isolated network for such devices. Meanwhile, devices are right to not allow MITMing their traffic and to treat that as a security hole, even if a very tiny fraction of their users might want to MITM it to try to do adblocking on a device they don't trust or fully control, rather than to exploit the device and turn it into a botnet.

Along similar lines, a security hole you can use for jailbreaking is also a security hole that could potentially be exploited by malware. As cute as things like "visit this webpage and it'll jailbreak your iPhone" were, it's good that that doesn't work anymore, because that is also a malware vector.

I'd like to see more devices being sold that give the user control, like the newly announced GrapheneOS phones for instance. I look forward to seeing how those are received.