I also lurked for a few months before registering and starting to post about a year and a half ago.
The big reason was time.
It simply takes more effort to read threads with the intention of responding than it does to skim them. You read the thread differently -- Example: Might want to make sure that you're not saying exactly the same thing that someone else already said.
On that note, I'd say the majority of value in a thread comes from reading, not posting, for most people who are just looking for perspective or guidance on a certain issue. In other words, people stop by a thread, learn what they need to learn, and move on.
So if you have, like, 20 minutes to cruise the forums, you might have a choice between reading 10 threads without posting replies, or reading 3 and posting replies. For the sake of your own financial learning efficiency, most people go with the former and read as much as they can.
All that said, Cheddar, I completely agree that the more people post, the better. Just do it - we're all the better for it.