Well, I don’t count as a lurker anymore but I only started posting about two weeks ago and lurked about 2 years before that so I think I qualify to comment about it. The only 2 forums where I post(ed) at all are LessWrong and INTPCentral.
INTPCentral was more of an experiment to see if I could sustain posting for an extended period of time. It didn’t work and after 2 weeks I lost interest. LessWrong has less chance going the same way because of the high level of most top posts. That’s my first barrier to post. The online community has to be interesting enough to make me come back.
The second is a certain reluctance to comment at all. I think that has to do with my aversion to attention (although this doesn’t fly when I’m with friends. Then I have no problem with it). The only reason to call attention to myself is when I can significantly add to the conversation or to correct someone. That also makes it difficult for me to comment on a top level post that already has been thoroughly analyzed in the comments. Adding a comment that doesn’t add anything does look too much like yelling ‘me too, me too’.
If understand you correctly, you are saying that most people are not knowledgeable enough about the different domains in question to make any (or judge any) cross-domain connections. This seems plausible.
I can think however of another argument that confirms this but also clarifies why on Less Wrong we think that people actively compartmentalize instead of failing to make the connection and that is selection bias. Most people on this site are scientists, programmers or other technical professions. It seems that most are also consequentialists. Not surprisingly, both these facts points to people who enjoy following a chain of logic all way to the end.
So, we tend to learn a field until we know it’s basic principles. For example, if you learn about gravity, you can learn just enough so you can calculate the falling speed of an object in gravitational field or you can learn about the bending of space-time by mass. It seems rather obvious to me that the second method encourages cross-domain connections. If you don’t know the basic underlying principles of the domains you can’t make connections.
I also see this all the time when I teach someone how to use computers. Some people build an internal model of how a computer & programs conceptually work and are then able to use most basic programs. Others learn by memorizing each step and are looking at each program as a domain on it’s own instead of generalizing across all programs.