Because looking good on paper is not the same thing as being good.
When hiring a professor, nearly every college uses commonly agreed-upon criteria. Among these, perhaps the most important is whether the applicant has a graduate degree.
On one hand, credentials are a critical part of a school’s brand. Given that students are coughing up an arm and a leg for today’s tuition, it’s helpful when a school can boast that “every single one of our faculty holds an advanced degree.” Indeed, this percentage contributes to a school’s ranking.
This argument makes sense, especially from a marketing perspective. Yet it’s less compelling when applied to adjunct, rather than tenure-track, professors — i.e., those who teach as a sideline. We adjuncts typically have another job that pays the bills; we don’t teach for the money, but because we love doing it.