Density. Certainly it's possible to start a courier business in a less-dense city (and not saying there aren't others -- Boston and NYC especially) but you've now made getting off the ground a lot harder. Obviously the culture in SF has made courier start-up pretty popular lately, but without density I bet that we'd be seeing far less of them.
Density allows these P2P start-ups to succeed with a lot less hassle, and from there they can grow the capital they need to expand.
Density allows these P2P start-ups to succeed with a lot less hassle, and from there they can grow the capital they need to expand.