I think you are absolutely right that there appears to be some low hanging fruit for tech companies to pick by looking to "second tier metros". But I inertia and (lack of) critical mass play a big role.

In the past decade (i.e., when the money was plentiful) when a startup is young, the TC of its engineers rarely makes or breaks the startup. Being able to get an MVP out and iterate quickly is more important, so it was a rational choice to stay in the Bay Area even if it means 30% inflated TC. And after that moving is expensive in both time and money and risky (e.g., a key engineer might not want to go).

And having a critical mass of tech companies helps attract talent: if a company goes under or has large layoffs it is perceived to be easier to find a new job in the center of the tech hub.

I think covid helped nudge along the process of moving tech development out of SV, but it is a slow process. My 2c.