But if you work in tech your rent barely makes a difference?

If you are a senior engineer and make 350k$/year (Meta is more like 500k$/year) and you pay 5000$/month for rent and could potentially pay 2500$/month instead in a MCOL, that's only 30k$/year of savings? Negligible compared to your income.

And on top of that most companies will cut your income for moving to a MCOL/LCOL by more than those savings.

If anything it is an argument to stay in SV!