The problem is that the Valley is a host of municipalities of differing sizes, with San Jose at the bottom end. It doesn't really go as far north as San Francisco (yet). Westchester works as it does because it's largely feeding into New York's superb infrastructure -- there's no comparable network (or city of comparable size) on the Peninsula. There are a lot of people commuting from e.g. Belmont to Cupertino or Mountain View to Palo Alto.