Looking at another profession can help see NTFB's point. There are a bunch of teachers in my family. If they move from one school district to another, they are normally hired at the same "step" they were on with the previous employer. This allows teachers to move freely from one location to another without losing pay.

Of course, training costs are not an issue in that profession. There is a significant cost to mobility in the aviation industry, thanks to FAA regulations that require a pilot with 10,000 hours in a 737 at Company A to go through the same training as a pilot with none when both are hired at Company B, and an FO with 1,000 hours in the plane to go through the same training again to become a CA.