Do you know how many Ph.Ds in computer science are granted each year? The last statistic I saw was, around 800. A good percentage of those go into teaching. The supply is lower than the demand.
how many people want to teach? and who are they teaching? mostly foreign nationals now.
no offense, teaching is a noble profession, but its not a productive industry. these CS PHds should be working at US technology firms - but the demand (and pay grade) for them in those jobs, isn't there.