The real reason is that there are too many tech companies that refuse to build equivalent headquarters in other cities. It's concentrating the wealth of companies that gather money from around the world in a tiny area, and that by definition will create inflation.
If Google, Facebook, etc, created equivalent headquarters in Kansas City, in Cleveland, etc (as in, they have equal say as the main HQ and just as good projects), then I believe the wealth could be spread over the entire country or globe, and everyone would benefit.
People NEED to move away from Silicon Valley. Having so many rich, tech workers living in a small area is toxic, just like Wall Street and Manhattan is toxic.
Once the pressure of adding thousands of 6 figure engineers coming to the Bay Area recedes, then housing will fall to a more rational level.
I've been living the in Bay Area since the dot com boom. Housing prices were always high, but never this toxic, until frankly the FANG stocks started breaking out in around 2011/2012. You can see this in the stock market as well. When these global internet companies started consolidating their power and consequently their wealth, it created a mass migration to the Bay Area of tech workers who were paid well above the average, and caused massive inflation.
You want to start a company? Go somewhere else. Be a part of the solution, not part of the problem.