You don't see graffiti in rich neighborhoods either because you're describing the suburbs where nobody really lives (as a measurement of people per square kilometer) or because rich neighborhoods get immediate attention by cleaners (or the rich hire private cleaners).

There's plenty of graffiti in Manhattan, have you looked up how much it costs to rent there lately?

This is an interesting point, but I push back a bit. First, "Manhattan" is much larger than most people realise. There is almost no graffiti in the rich neighborhoods (Upper West/East, etc.), but there is plenty of graffitti in the more iffy neighborhoods (East Villiage, ABCs, SOHO, etc.). It pretty much scales with wealth -- richer has less graffiti. Second, specific to this post about San Francisco, there is almost no graffiti in the suburban areas out west (Sunset, Richmond) and wealthy neighborhoods like Russian Hill or The Marina, but loads of graffiti in The Mission, Potrero Hill, and SoMa.