Presumably because in the case of the South London Metro routes (whatever that is as it only seems to be an operational internal timetable planning convenience of Southern/GTR but not anything that is marketed on the ground any longer to passengers these days and it also makes no sense that services to any stations beyond Epsom are being caught up in it not least because Dorking to Horsham is only hourly) almost every single timetable since Southern took over as the operator has made services take longer and longer and longer to reach their destination (as in a 51 minute journey from Ockley to London Victoria has gone up from 51 minutes to 76 minutes in 30 years with 11 minutes more added to the fastest possible Ockley to Victoria service at 1624 on weekdays just by these latest September 2022 revisions) and added in more and more stops along the route (with Mitcham Eastfields, Micham Junction and Hackbridge now all part of the route all the time).
This is a proper South London Metro proposal at
https://districtdavesforum.co.uk/thread/30169/tube-south-london-metro-proposals and on the line to Dorking and Horsham (I don't know if this would also have seen South London Metro as part of the Overground) and on my line it doesn't run further south than Cheam as well as not including services to Waterloo from Dorking or to Victoria from Dorking and Horsham in the scope of the South London Metro at all. Or this is a different theory of what the South London Metro might be at
https://anonw.com/tag/south-london-metro/ And hard to read this Southern Elliptic map at
https://londonist.com/2013/09/an-alternative-map-of-south-london-train-routes but it definitely doesn't seem to run as far south as Dorking.
Can't finding anything about South London Metro on Southern's own websites at all these days. South London Metro posters and branding used to be used everywhere 20 or so years ago although still Epsom to Horsham was definitely never part of the South London Metro concept except that the stations south of Epsom happened to be served by trains that also ran through the vague and amorphous South London Metro area.
Perhaps I need to discuss all this (by email given that there is no longer any monthly Southern/GTR Meet The Manager session) with the local Area Manager, who's name has just appeared on the latest station specific poster at Ockley Station.