There are plenty of companies in the tech industry who already have done this - how many times have you heard about someone told by their employer that they will have to relocate to X and take a pay cut, or lose their job?
These are, however, companies whose M.O. is cutting costs to the bare minimum to increase margins. You can probably think of many that fit this model. This nearly always means they are no longer focused on growth or product innovation.
Even growth-focused companies have already moved a most of their operational and support to lower cost-of-labor areas, well before COVID19. But very few tech companies have their creative functions in those areas.
It is not just quantity of population, but characteristics of the population.
As an example; in Boston, if you are looking to setup a Biotech shop you need to throw a rock down the street and you'll hit someone involved in the industry. In Louisville, KY? Forget about it, yes there are some folks who are in the field but as a whole?
Meanwhile, the bay area is the #1 highest COL in the continental U.S. Phoenix is ~#29.
If the argument is that tech requires a big city, I think Phoenix qualifies. If this is a hangup for you, consider Philadelphia: 5-7 M people, #21 COL.
It is obvious to me that population size is not why tech giants gravitate to SF or Seattle. (I'm confident that the high COL is also not the reason.)
Tucking 1000 employees into Houston and 1000 into Charlotte and 1000 into Toronto, then saving $200,000,000 per year seems very attractive to me. And if relocating 3000 makes sense, why not 10000?
There's obviously enough good reasons that they haven't done it. But from my seat in the balcony of the ignorant it seems awfully strange from an industry that prides itself on disrupting stale thinking.