Erin cites this view of why people get Ph.D.s from Grant McCracken.

This apparently irrational moment of consumer behavior, the purchase of a Ph.D. that will not bring employment, is actually knowing and deliberate. It is an act of self and world construction. It allows the individual to make certain claims to identity. It allows them to build and to occupy a certain understanding of the world.

This leads Erin to comment:

Looked at McCracken's way, the degree is a kind of affective license, a lifelong permit to make certain kinds of pronouncements about the world, and to excuse--even dignify--behavioral patterns that tend not to result in the most stable, secure, or happy lives. McCracken's analysis may look cynical on the surface--but it may be more accurate to say that McCracken is proposing that the pursuit of the Ph.D., at least in the humanities and softer social sciences, is itself a deeply cynical act.

I assume that "softer" social sciences is a way to let us economists off the hook, but I nevertheless think it's accurate even for us. Economists do like to discuss unintended consequences: they'll move an argument forward that makes policy X look like it's good, and then slam you with an unintended consequence that makes X look bad ... and sometimes will reverse again. An economist will always talk about "that which is seen and that which is unseen" even if they are not fans of Bastiat. It's hard to get your argument taken seriously when you lack a credential or marker of some sort.

But that said, one doesn't have to spend too long in graduate school in the social sciences or humanities. Most of my friends in Claremont were either econ or philosophy Ph.D. candidates (and a few government majors), a world I'm glad to have left behind.