Simple. You keep getting emails from recruiters advertising $50k/year Senior Developer positions for the same reason that you get 200 resumes from completely unqualified people for every position you advertise.
Good positions fill quickly, just like good developers find jobs quickly. What's left is bad developers and bad jobs.
So what you're seeing is not hundreds of employers who don't know what the market rate is. It's dozens of employers who refuse to believe the market rate is where it is, and therefore continue advertising for the same position over and over. There will always be that $50k Senior Dev position available in Hillsboro because they're never going to fill it.
Good positions fill quickly, just like good developers find jobs quickly. What's left is bad developers and bad jobs.
So what you're seeing is not hundreds of employers who don't know what the market rate is. It's dozens of employers who refuse to believe the market rate is where it is, and therefore continue advertising for the same position over and over. There will always be that $50k Senior Dev position available in Hillsboro because they're never going to fill it.