There are market research firms that specialize in this area. It's useful to have such information, so it has monetary value - money presumably saved by underpaying or avoiding overpaying your workers. It's not something the average employee would afford in money and effort to research.

It's a good question how the data is obtained. Maybe through mutual data sharing agreements in exchange for access? Anyway, I've see such reports for my country, with very detailed breakdowns for job types, seniority, broad technology area, forecasts etc.