There is also a cost to employment besides salary. Every state has their own laws and orgs to register to. (And every country has different social insurance taxes.) This means not only needing to hiring a lawyer to make sure they are in compliance with local laws, but paying an administrator to manage it.
It is not just the employer pocketing wages in LCOLs. There is a cost.
For larger companies; eventually they build out to a point in their main hq or want to provide jobs to get the support of that areas politicians in Congress; that these additional costs for hiring in another state are outweighed by the benefits.
But this can be very prohibitive to startup or even medium size companies. Especially 'slow-growth' ones that do not take VC funding and do not have access to a VC's network of lawyers.
If I move to Washington, with no income tax, but does have a revenue tax(B&O); does my remote employer now have a business nexus within that state? Do they have to pay that B&O tax that Washington levies (is it a tax for national revenue or just from all revenue within the state)? Not a lawyer.
Taking a salary cut, when you also take a bigger cut to inflated property values and taxes; is not necessarily the employer taking advantage, or even a net gain for both sides.
A better argument to me to staying in certain areas like the SV or NYC is losing the ability to easily network and find another company that would employ FAANG level skills. This ability is naturally diminished during COVID and I imagine many companies will take steps to keep their work decoupled from their offices in case another pandemic breaks out. I still do think about it as housing is just prohibitive here in CA where there are local tech jobs.