A car can be parked on the street (at least in theory, I'm aware SF's parking may be less amenable), and has a high likelihood of being there and intact when you return.

Without bike-parking infrastructure -- ubiquitous racks -- bikes are far less likely to be found where left, and far more likely to be damaged, vandalised, and/or have components nicked.

I've lived where cycling was well-supported and routes and racks reasonable. And where not. For SF, hills, parking, and traffic all work against cycling.