At the moment, there is a surplus supply of PhDs. If, at some point, that gets corrected, things may change. Though I should note that PhD students in history

At the moment, there is a surplus supply of PhDs. If, at some point, that gets corrected, things may change. 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.