Sounds like a death spiral. My father got a PhD in history so he could be a professor. If he couldn't expect that, he would have gone to business school, reducing the demand for history professors and so on until there's none left. You get it.
Though I should note that PhD students in history aren't major drivers of demand for professors of history - undergraduates are.
In the sciences, there's a bit more of an incentive to have lots of graduate students.