I am curious about it as well, Steve. I don't know why they do this. I have no idea why a real person would take the trouble of signing up for a site and never posting a single thing. A bot is doing it to open the door for its spam activities. Perhaps some get through the door but the software cuts them off the rest of the way. Not real sure. But hey Noelle, if you are really a human, I invite you to prove my theory wrong. Just post one thing and then return to your aloof silence.
Also, while we are pondering inexplicables, we mull over this one:
(I have obscured IP addresses out of courtesy)

How is it that guest users are posting? They don't even have an option to post unless they are logged in (and of course we never see these posts). This is consistent with what you would expect of bot activity (an automated script that is crawling the internet and trying to spam sites wherever it can). Fortunately the software must be blocking most of these attempts. Well, maybe these few are just the bots and rest are real people. But I am curious who these people are, where they are coming from, and why they are such loyal readers yet never sign up, sign in, or post.