There are a lot of reasons to not background scan, like latency and packet loss - scanning for networks typically takes a few hundred milliseconds and in that time you cannot communicate with the network you’re connected to. If you don’t have the network manager open looking for a network, and you’re connected to a network, background scanning is usually disabled.
When a network with multiple APs or bands is connected, neighbourhood reports inform the device of the neighbouring SSIDs so background scanning doesn’t have to occur and interrupt connectivity.