I think labour markets are some of the least efficient markets that exist. Without immigration it takes at least 4+ years for extra supply to become available based on increasing prices. And when you hire someone you really don't know what you're getting, particularly in software.

Constraining supply increases salaries precisely because it takes supply so long to incorporate information.

On top of that, supply and demand is a pretty leaky abstraction, I don't expect it to be very accurate in such a nonideal market.