That's nice of you to say, but I think a few things play into it:

1) I've found most FE devs are not classically trained software engineers (most don't have CS degrees)

2) There's still a stigma that all FE is is writing HTML, CSS and JS (when, in fact, there's now entire build management layers, and the frontend stack is as complicated and nuanced as that of any OS' SDK). This is a holdover idea that is starting to diminish.

3) Because of 1, the market likely has more FE developers available to it. I'd be curious to see what your typical self-taught Java, C#, Python, Ruby or DBA makes.

4) I've also noticed a trend of BE engineers showing interest in front-end work, given its new challenges and such, further saturating the market. This is completely anecdotal though.

It might be that some companies offer near-SF salaries to make it a slam dunk for the new hire. From the point of view of a Valley giant, they probably churn higher-salaried people in SF/SV all the time, so one more located elsewhere is not a biggie. At 10% less it's still a win, and because of the differential from local prices, the new hire will be super excited to join and stick around, so why not? The more you push down towards average local salaries, the more you have to compete with local businesses and the more you risk getting average performance out of the hire, all for a relatively small cash saving.

doh. It didn't occur to me people would stay in Seattle after getting Google jobs.

Perhaps Google should COL-adjust SF employees up to about $500k/year.

There's a weird life trap where you're making what most people would consider "a lot," but over half your take home pay is eaten by rent and food that costs 3x what it would somewhere else. So your "a lot" turns into "can't afford a car" or "can't save up enough to get out of the cycle" without moving to less desirable places.

Better to serve in hell (SF/NYC) than reign in flyover country?

Note that a $5k/mo mortgage means a $1.2m+ selling price, for ~2000sqft. That's still insane (and doesn't include property tax, maintenance, utilities, or any of the other homeowner costs). Imho, anyone is crazy who thinks spending half their take home on a mortgage is acceptable.