I've found that this is often because someone told them that it was an important document. The only thing important about your dissertation is that you complete it in a reasonable amount of time and effort. It is, by almost any account, the least significant major paper you'll ever write, and so it should be: it's your first!
It's similar to the old "Q: What do you call a guy who graduates in the bottom third of his class from med school? A: Doctor!"
/jordan