The same people who make 300-400k in San Francisco, also do so in Seattle. I don't know where your getting your information from, but all existing stats disagree with you for a reason.
I have hired many people who worked for Seattle BigCos, and I know many people who have gotten offers from Seattle BigCos trying to lure them away from the Bay Area.
And I’ve chatted with people doing competitive analysis on what offers to make to engineers.
The Bay Area generally has 20-30% higher total comp after the first 2-3 years.
Somewhat amusingly, the headline salaries are pretty similar, which makes people think total comp is similar. It’s all about the bonus and RSUs.