Because you're around a ton of people who are interested in the same thing as you are. Caveat: If you're not interested in the things SF engineers are interested in, that means you're surrounded by masses of incredibly boring - to you - folks :)
Because that introduction you need to make things pop is super-easy compared to other places.
Doesn't mean you _have_ to start in SF, but for certain classes of ventures, it's the place that makes it the easiest.