Ph.Ds In Public Schools

Why Aren’t More Ph.D.s Teaching in Public Schools? - Laura McKenna - The Atlantic:

...American universities award more than 60,000 doctoral degrees every year. However, there are not enough academic jobs for all those graduates. One study asserts that only 41 percent of Ph.D.s will find tenure-track positions. Some studies are slightly more optimistic. In a report for the academic journal PS, Jennifer Seagal Diascro reported that 49 percent of the 816 Ph.D.s who graduated from political science programs between 2009 and 2010 found permanent academic positions.

I'll offer a reason not mentioned in the article: lack of respect. People who earn terminal degrees typically are highly motivated and very intelligent, thus they are less likely to develop the patience and philosophical inclination that teachers need to thrive in K-12 and far less likely to suffer fools kindly-- they expect people to listen to them.