I've been a member here for 7 years, and I was a lurker long before that. I never posted much, but I've been here consistently and have seen a lot of guys come and go.
I don't think anyone really disappears for good, they just get tied up with other things going on in their lives. A lot of veteran posters have had children, job changes, moves, military time, battles with cancer and a host of other things that easily take precedence over arcade games.
Other times, people just start lurking more and posting less, avoiding the posts that we've seen 20 times over the years that argue about price, legalities, and whether or not something will, in fact, fit in the back of Subaru Outback. I think some of those guys have answered the same question so many times that they just figure it's time to let others step in and start the cycle over again.