The article doesn't really touch on this, but for people embarking on a PhD program, school is what they've been doing 8 or so hours a day for 16+ years and school is what they're good at (or else they wouldn't have gotten into the PhD program). It's the least disruptive path post-college for a certain group of college grads and their choice is to trade potential financial gain for stability and familiarity. That may not be the best choice for each one of them, but it's certainly not irrational.