Routers are decently large , generally have enclosures, and are meant to be placed in a reasonably accesible position for those who should have access to them while at the same time out of sight for those unauthorized, which makes putting a sticker on it, keeping it there, and having the right people read it when needed is trivial.

Some IoT devices could be handled the same way, but there are plenty of reasonable IoT applications where a password written on the device is impractical or a security risk.