I believe there are several reasons for this:
As the search box often shows, a lot of the users are foreign. (I myself have muddled through making an account on a Russian forum or two when either I thought it was something else, or there was only a foreign source for what I was looking for at the time, such as a Direct 3D re-render of Frontier: First Encounters). Some are just people who are looking for a copy of the game and might be mistaken in thinking that you have to register to get a download link, and I'm sure there are a select few bots that are able to make it through the registration process, but are either too new or too old to figure out the posting functions. Of course this is all speculation, but its what makes the most sense to me.