Excellent question. One also encounters men who voluntarily chose to remain privates, or having held higher rank, opted to return to being a private. Less work, less responsibility, more social interaction (being a conscientious NCO or commissioned officer who properly avoids over-socialization - that diminishes discipline - can be a rather lonely endeavor). There were plenty of well-educated privates; I have found that one out of every dozen college alumni who served in the eastern armies was a private as of summer 1863.