You have to see this from the point of view of a middle-manager whose main concern is his own career and salary. First, it's much easier to manage junior than seniors, more so when you are not an engineer yourself. Also, it's more likely you get a better salary when managing a large team of junior than a small team of senior. Even better, you can become a manager of managers if there is a bunch of juniors to manage. Another problem is that an employee cannot earn more than his boss, and non-tech managers salaries cap at a certain point.