Because where I'm sitting, facebook, Google, Amazon, Microsoft, and Apple, which are 5 of the top 6 most valuable US companies, are HQ'd in the Bay Area or Seattle. So if you want that caliber of job, with the opportunities and pay ($200-300k+ for mid-level people), you really do have to live in SF/Seattle.
I don't work a BigCo job for other reasons but the reasoning does seem sound.
Amazon recruiters have also been pinging me for jobs they have in SoCal, although I wouldn't say that LA is significantly cheaper than the Bay Area or Seattle.
while i'm commenting, i'll note that i don't feel boulder really competes with NC's research triangle, or austin. boulder county population is 310k, and a sizable chunk of that is tied up in and around the university and government research facilities that i feel don't impact local tech industry that much.
Home to UNC, Duke, and the somewhat famous Research Triangle Park, a suburban campus containing R&D for Red Hat, IBM, and several big pharmaceutical companies.
The Palo Alto of the South.
I used to live in South Florida, and I bet I could exhaust the search space of "tech jobs south of Orlando" in about two weeks of interviewing. After that, you're looking at a move. It's not like the same companies will interview you over and over until you pass.
I mean, yeah if you move to rural South Dakota you won't find much, but there are plenty of growing tech centers like Raleigh, Boulder, Austin, etc. that have much lower cost of living, but still a lot of availability in local tech jobs.
Also to note, the 3-9 month job search should be much more sustainable in Nowheresville, whereas in SF you are going to be screwed if your job search is extended.