Part of it is the frequently-found inferiority complex among overwhelmed, but competitive, graduate students. If they can pass that exam, then surely I can too. If they can get into that program, than surely I'm qualified to do that. They must be so competent if they can read that in Old English! That kind of thing.
It's kind of funny, I'm no longer even at a formally competitive part of being a student. It's me against myself and my ability or desire to work on my dissertation topic. No one else is doing it. (I am so glad my research life isn't remotely like that in Possession.)