Silicon Valley seems cheap compared to London
That's fairly distinct from SF's problem (lots of people wanting to live there, but artificially constrained supply, both construction limits and rent control)
While I'm sure a ridiculous proportion of the v high wealth areas are laying vacant, in areas like Hackney (which are the fastest growing) I very much doubt that is the case. These flats don't make up much of the London housing stock, %age wise.
I.e. property speculators. There's plenty of speculation in the Bay Area too.
> artificially constrained supply
Thanks to lobbying from people who benefit from constrained supply ... e.g. NIMBYists and speculators.