I think the point is a huge number of tech jobs are within these locations. The SF bay area has something stupid like the top 3 cities when it comes to tech jobs. Throw out all the locations with the most jobs and surprise, salaries suck.
Well, I wouldn't say salaries "suck" outside of well-known tech centers like SF/Bay Area and NYC. They aren't great, though. Even in the Bay Area, at least, the salaries of engineers still pale in comparison to the salaries of managers and executives.
