You are assuming that tenured positions become available at a steady rate. They don’t. There are waves of retirements, like the baby boomer generation retiring in the next few years. Engineering schools grow and shrink, further complicating the picture. In addition, the trend toward hiring more lecturers to teach has also negatively affected tenured positions. Finally , positions in industry, finance, and consulting pay a lot more so the supply of Ph.D.'s who want to go through the hardships of obtaining a tenured position is relatively sparse.