Note: This is not a scientific poll.
I don't really get why anyone hasn't. It isn't difficult. It isn't time-consuming. The most "probing" question on there asked what race you are (for which I was tempted to write either "human" or "Boston marathon"), which most people could pretty easily figure out just by looking at you, if they cared. The census is confidential. So, the only thing I can figure is that people are either lazy or paranoid!
WordWiz78 makes several good points, however, as far as probing goes, I felt that the most probing question for me was whether or not my home did or did not have a mortgage. It is none of the government's business whether or not my home is paid for. As long as I am paying for it (taxes and/or mortgage), what business is it of the government, or anyone else, for that matter, as to the title and ownership of my home? Would you like it, and answer a perfect stranger if s/he asked you if your home had a mortgage? Talk about intrusive, and downright nosiness!