Baby boomers or not, the number of PhD graduates far exceeds the number of professors due to the simple logistics of teaching.
Suppose you start a professorship at 30, and retire at 70. How many PhD students do you advise per year? Let's say 1.5 just to be on the low side. And suppose they each take 5 years to graduate. You just cranked out a dozen PhDs, and created one faculty opening by retiring.
One should expect an advanced degree to increase one's job prospects, but it's numerically silly to expect, specifically, a faculty position. This is why every university hires people with degrees from an even better university---not because NIU frowns on NIU grads, but because the market for the teacher's job is so competitive that only the best CVs get in.