There is a lot of friction in the employment market, and market participants are a long long way from rational actors, so labour arbitrage does not really work like it should, and supply & demand economics only really applies in extremis, over extended periods time.

As a result, price discovery in the employment market is extremely inefficient - and tends to be set using cultural and cognitive biases; perceived value; and so on, rather than with rational market considerations.

Many employers simply follow the herd when setting salaries - offering the median of what everybody else in the marketplace offers - or they will set a salary that is in alignment with their cultural preconceptions of "value".

On the other side of the fence, it is rare to find a developer who will negotiate for a higher salary, or reject a job offer choosing instead to wait for a better role to become available, a factor that tends to reduce the effective "liquidity" of the jobs market.

So, it is entirely plausible that salaries spend a long time stuck far far away from the equilibrium point that they would reach if supply/demand economics really did rule the jobs market.