Sure, they have to "import" employees, but importing 10% of the workforce in the Bay Area at X cost is apparently more appealing than importing 98% of employees to Tulsa at Y cost.
(Relatedly; Tulsa's not exactly a small place, but if you think the influence of "big tech" is bad in a larger metro like the Bay Area, I'd HATE to see what it would look like to bring a deluge of 'outsiders' into a much less populous region)
A tech firm in Boise might not be able to hire and Elixir developer at any price.
In SF, there will be employees as long as you have cash. Not so elsewhere.
There are plenty of businesses that don't need to be on fancy infrastructure or use the coolest new language for their backend.
I think the timescale matters. If it became known over a period of years that some company was paying $1mm for Elixir devs in Boise, loads of folks would consider learning Elixir and moving to Boise.
- There are already a ton of existing technology companies in the Bay Area, and also a ton of employees.
- Students from schools around the country who are interested in tech might plan to move to the Bay Area due to the number of companies there.
- If your company doesn't hire in the Bay Area, it may miss out on the chance to hire the employees in the groups above.
The end result is an abnormally high supply of technology workers in the Bay Area. I'm not saying that's something intrinsic to the location, it seems to me it's more a result of recent circumstances.
I wouldn't be surprised to find out that 90% of the reason FANG companies operate in large metropolitan areas has almost nothing to do with employees and everything to do with global image.
If Stanford or another incubator of tech innovators was in podunk MT, then we would be discussing that location instead.
When Intel acquired the AI startup I worked for a couple years ago, they built us a downtown office, much bigger than our team currently needed. Why? Because there are lots of AI people in Seattle who will not move to California, and Intel wants a chance to hire them. But it's still a very small office compared to Intel's global footprint; it does not affect their "image" at all. It's just a practical way of gaining access to Seattle's engineering talent.
It is a simple virtuous cycle. The presence of good jobs draws people who want to work in the field, and the presence of people with experience draws the companies who want to hire them. Geographic concentration is efficient.