I think this is a function of the relative immaturity of software development. The pay is lagging behind the importance and scarcity of software developers, and

I think this is a function of the relative immaturity of software development. The pay is lagging behind the importance and scarcity of software developers, and this is distributed unevenly. Company A may be in a place where the relative value of their developers may be 2% more than what they currently pay them, but for company B is in a place where they would gladly pay 25% more and still get a big fat return on that investment. By the time company A catches up (as in, software has now become way more important to their business), half of their developers have moved on. They review their recruitment and retention strategy, and one year on they are the ones paying more than company B.