There's an inherent information asymmetry in the job market too. Think of great coders who grow up in the middle of nowhere and get a job in the nearest city. They don't know what the vets at their company in Birmingham, AL are making. All they know is that the coders on HN all say "at least $100k for a dev in the bay area".

You need to do a lot of footwork and networking to build an accurate picture of the employment market in a region.