Initially this was true, certainly SV would use higher salaries to draw developers and engineers to the valley. But cost of living has exploded since then. A modern developer living in SV does not command a higher standard of living as food and housing costs are significantly higher than most of the rest of the country. A similar trend plays out on the east coast so it’s not just SV.
It’s balanced by the CoL adjustment. In short, SV engineers are not better off, and haven’t been for the past decade. This is apparent by the observed exodus in the light of Covid forcing a shift to primarily remote work. Engineers are trying to reclaim their wage advantage by moving to cheaper locations, and companies are fighting back by reducing compensation accordingly.