If this ever made sense, it makes little sense today. The more degrees, the longer your tenure, the more you make. As Al Shanker used to point out, there is no way that a mass enterprise like teaching will ever command across the board high salaries. High salaries for some, but not for all. To be sure, there are higher and lower teacher salaries, but they are largely the product of regional market differences.
Local wealth makes a difference as well.
But by and large, teacher pay is uniform. Conceptually the whole system makes little sense. For example, the assertion that teacher pay is too low is hard to support from an economic standpoint. So long as there are teachers applying for jobs in the profession, too low sounds more like a rallying cry than a comment on practice. As Chesterton so wisely said, tradition is the democracy of the dead.