London Overground do not have a particuarly large contingent of Inspectors, as far as I know and on the Southern Metro they got rid of all the Inspectors a few years ago now and made them all OBS on Mainline services. So fare evasion is very rife. Most fare evaders know this and will simply buy short tickets, or push through as they are not going to get challenged at the single staffed gateline at the other end for obvious reasons. Metro services seem to just be something operators have to run alongside the more lucrative mainline operation, which most of the resources seem to be focused on. I would like to see Metro operations split off from both Southern and Southeastern even if they are not going to spend big.
LO stations are staffed all day long with barriers on all exits in contrast to SE metro so on-board checks less important.
It doesn't stop the determined but it does stop the casual fare evader.