It's not like you can easily detect a rogue RPi with just nmap. It's trivial not to respond to anything sent to you. You have to start looking at ARP, but that's not iron-clad either.
But that is not the point, point is they make admins do stuff manually. Getting kids to brush teeth every morning and evening is hard, getting bunch of IT admins to do something, what seems pointless, every day is close to impossible. Setting up something that scans network every day is trivial by comparison.
https://en.wikipedia.org/wiki/Arpwatch