There are quite a few cameras that record to a device on the internal network - put them on a VLAN that has no outside connection and you have it all. That is quite an advanced setup though and it's good practice to save the recording at another location as well.
LAN over electricity wires is a proper pain in the ass. It's not like you can plug in a "router" to one outlet and have network in all others. It only goes to the group that particular outlet is in, so unless your wiring is made in a particular way, which existing houses aren't since it's not an established standard.
Well, what is stopping people for creating switches that go into the distribution panel? They can even fit as circuit breakers, so no change at all is necessary.
One would need a large diversity of products to make it work, what puts it out of reach of any small manufacturer. But why the large home automation sellers insist on their "you must add this huge panel, pass all those wires around, and can't retrofit any new device once the layout is settled" technologies is a mystery to me.
LAN over electricity wires is a proper pain in the ass. It's not like you can plug in a "router" to one outlet and have network in all others. It only goes to the group that particular outlet is in, so unless your wiring is made in a particular way, which existing houses aren't since it's not an established standard.