College lecturers hate students that don't turn up for class.
Students hate turning up for a boring class.
If the lecturer was more engaging, more interesting and made the class into a close knit-group – students would turn up more often. I know there were classes I never skipped.
Lecturers compete against television. If watching television is more interesting than attending the lecture, television wins.
Lurkers are a lot like college students. It's easy to hate them. They come, they view and they leave. If they having something more interesting to do with their time than participating, they wont participate. If watching television is more interesting than participating in your community, television wins.
It isn't lurkers fault that they lurk, it's ours. If your community was more engaging, more welcoming, more fun and easier to participate in – you would have less lurkers.