I guess they're flying down the peninsula and then turning back up to the airport, so other traffic inbound from other directions can take 28R without paths crossing
That's generally how SFO channels its traffic on a typical good-weather summer day, there can always be exceptions to that rule. If you look at many summer photos of parallel approaches at SFO you will typically see a larger (foreign) widebody aircraft in the foreground (rwy 28L) and some smaller domestic aircraft on 28R in the background.