This safety net is what keeps me here. It's not that San Francisco is more creative than Portland or its culture more exciting than New York or its cocktails better than Boston. I'm here because I was already a crazy person with very little aversion to risk who is bad at saving money. I could live almost anywhere but I'd eventually put myself into a bad situation where I'd have to take a job I didn't like or suffer through a prolonged period of unemployment finding the right place.
Pithy explanation for why San Francisco and the bay area continues to develop startups at an ever increasing pace -- living risks for talent is low.
This does not delve into the supply/demand relationship though. Good talent is still incredibly hard to come by and the city still loses good people because of its incredibly high living costs.