There are many reasons people lurk, as Phil listed. I've been on many, many forums myself anywhere from planted fish tanks to aquariums, cichlids, fish, local club forums and even a few political ones. Talk about hot button issues there! Politics is the hands-down definition of nasty, mean posts on a forum. I've been admin of more than one of the politically focused ones.
From both perspectives, being a newbie as well as an admin, I think most people lurk because they don't want to feel stupid and/or be "attacked" for asking what the more senior players on the specific forums consider stupid questions. I often will lurk and in fact introduced myself as a former lurker from Missouri.
Why did I lurk? I read and read, not only to gain information but also to learn the players on the forum itself. Who are they? How do they answer a repetitive question? Are they in general helpful and patient? Or do you get some nasty comment about using the search function or replying to an older post? So I have lurked several dart frog forums over the past few weeks and chose to join here due to the patient nature, abundance of information and the repeated posts by Phil saying there is no stupid or lame question as well as the fact that indeed most posts are answered from what I can see. It's kind of hard as a newbie to something to post a question just to have it ignored like it doesn't exist. Add to that some people just really have problems navigating a new site, as Phil also stated.
It is a friendly, open community that grows and educates and I think this forum has overall done a great job with that. There is a more respectful attitude here that gives a comfort level, it seems people can disagree respectfully and I like that. Some people just love to be argumentative and often they are the most vocal ones on any forum, it's refreshing to see a more respectful form of disagreeing here.