Again, you are assuming that everything has enough power to run a networking stack, or even that there is Turing-equivalent hardware to run anything at all. (Yes, yes, I know this is HN, where any device is fit for a computer; most of the world isn't.)
My water heater from late 1990s has a cleverly wired set of transistors - not even chips, the logic is literally wired into the product: cheap yet resilient. Connecting that to the internet would need adding some sort of computing device, adding several orders of magnitude in complexity (you can ping it and now it's also brittle, yay).
(I assume that you mean "to the local network, which is then connected via more conventional means, oh, and did I mention you need yet another computer to route data to the remote end?" - from what I've seen, attempts at WAN-over-power have largely been abandoned, even industrial IoT went for LoRa and friends. And from what I've experienced, LAN-over-power is a last-resort desperate hack: better than nothing at all, but barely better than two cans on a string w/r/t packet loss.)
I'm not assuming anything, I'm not participating in this discussion (and I agree with you). I'm just sharing an interesting information for the GP because it's not well known. You're right that it's not really usable in production; but it's nice for some simple home automation.
(I assume that you mean "to the local network, which is then connected via more conventional means, oh, and did I mention you need yet another computer to route data to the remote end?" - from what I've seen, attempts at WAN-over-power have largely been abandoned, even industrial IoT went for LoRa and friends. And from what I've experienced, LAN-over-power is a last-resort desperate hack: better than nothing at all, but barely better than two cans on a string w/r/t packet loss.)