Because they never take ownership of their lives, and don't overcome cynicism enough to do what I call a "romantic PhD." Most often this is due to a too-strong attachment to the social/community life and roles in graduate school, where there are certain ritualistic ways to play the various roles. "Graduate student" in particular, is supposed to be played with a slightly ironic, superior air designed to hide deep insecurities within. Naive enthusiasm can lead to social ostracization within the graduate student community, which can be a good thing, since "stable graduate student community" is by definition composed of those who are hanging around for far too long instead of moving on.

I wrote an article about this for the ABDSG email list ("All but the dissertation survival guide"), a very popular resource for PhD students stuck at the ABD level.

http://www.ribbonfarm.com/2008/0...

I finished my PhD on time, without disillusionment, completely happy with my experience and pumped up for a life of ideas.