alt Hacker NewsRF human detection sensors ((that can even tell you the heart rate of someone in the room (if its below 120 I think)), cost almost nothing. Or at least they did before tariffs .
They can also be programmed to detect people on the floor, so if you have elderly in your house you can know if someone fell, without cameras. They are made for hospitals but are cheap, but not 100% accurate for HR and falls, but reliable enough for security, and cheap.
RF would have to be placed in each room, no? And would require wiring or be battery powered.
Wifi is something most people already have available, and requires no new wiring or battery management. That's the selling point.