Reason #3: You Think Anyone Can Be A Writer
(jb, this one's for you!)
Putting 50,000 words in nonrandom order in 30 days makes you a "writer" in exactly the same sense that changing a hundred lightbulbs makes you an electrician.
This is NaNoWriMo's central conceit - the idea that you're a "writer" if you attempt their arbitrary challenge. They actually boast this in their FAQ. It's all about wearing the costume, the romanticised idea of a writer.
Whatever happened to wearing a cravat, not washing your hair, carrying a notebook full of scribbled haiku's and nursing the same cup of coffee all day? Now I gotta write something?