I wonder if the same people would have failed out of any job requiring human interaction. One thing graduate education does is attract people who know that they would struggle in a mainstream work environment for whatever reason. Some are outright crazy.

Quite possibly. I do know it's common to the point of being a trope that some graduate students are just in graduate school to defer having to enter the work-force. Some see it as a way to put off having to make major decisions or processes like job hunting. Not like there's no real reason some opt to do things like that. It is markedly easier to accept scholarships and do the grad school circuit than find a job if you have mid to high grades.