I mean, yeah, you're correct that there are many more potential technical employers in San Francisco. However, this doesn't necessarily correlate with those jobs being more accessible or desirable than the jobs available in other places.
For one, you have a lot more competition from other applicants, because every brogrammer thinks that he needs to move to SV to be part of the scene (and I really dislike that SV is becoming Hollywood-for-nerds; flooded with tons of fledgling wannabes who worship at the feet of an elite few).
For two, you have a lot more snobbery and people expecting specific pedigrees. They want Ivy League, they want other startup or big-name tech experience, etc. There are a lot more people who think talking to a candidate coming from a job in Nebraska where he wrote software for a company that runs a machinery rental business (for example) is below them. That type of experience is severely undervalued, and if SF needs anything it's more people with an appreciation for a stable production ethos.
For three, SF is appreciably worse along most axes and the extreme cost of living means the "extra" money go down the drain very quickly, seriously diluting the real value of the salary. This is compounded when you consider that taxes are also very high in CA and SF, especially compared with states without income taxes like TX or FL.
Four, we're talking about "the Midwest" as a unit here, but the fact is there are a lot of cities in the Midwest and some of them have a lot more bustle than others. There are not many companies that are strictly tech companies, but there are hundreds of potential employers in every metro area, and again, some are much more active than others.
Also, when discussing job markets in the "Midwest", I'm really referring to everything that's not a tech hub, because the distinction is really tech hub v. non-tech hub. Houston and Dallas/Ft. Worth are in the top 5 biggest metros in the US and I'm sure there's plenty of tech work there. Florida is one of the most populous states in the US and the availability will depend on the metro, but they have a good share of tech work. The Mountain West has much to offer (especially Utah).
The point is that there is plenty of opportunity for someone who wants to look outside of NYC, SF, SEA, and LA. No one should feel like they're trapped there as long as they're willing to work for some companies that aren't household names.
Lastly, you never run out of opportunities if you're willing to work remote, and this can be a good way to maximize the benefit of low COL areas.