Your point is quite reasonable in general but the discussion was for Silicon Valley and San Francisco. My GF has worked for Microsoft, Amazon, FB and now Google (and some startups over the years). How? Well they all have big tech offices close to Palo Alto. When she worked for various startups she was an ex-MS, ex-FB etc person.
If she'd stayed in Chicago (Northwestern grad), that's unlikely. Of course those companies have outposts there, but fewer tech roles by an order of magnitude or two.