I have noticed this too. Look at any thread from a couple years back, or page 100 of What is It. A huge amount of the people starting these threads haven't been active since shortly after they posted them. I can't count how many times someone posted something great, people asked for other photos, or some new information, but the person had already left the forum before they could add these things. I remember that shortly after I joined in February 2013, the number of registered members according to the home page was at around 50,000 or so. The funny thing is, if you look at any recent thread on here, very few people leaving replies joined before that date, and are not part of that 50,000, and 50,000 is a huge number! Out of around 50,000 members, there is a very small amount of currently extremely active members, part of that number. Perhaps people don't have the attention span to be an active member of a forum, or the interest, that has happened to me before. Or they might be part of another big forum, which happened to me when joining another metal detecting forum at the same time I was a member here. Maybe they are intimidated by some of the incredible things they see on the banner, and figure that they can never find anything that good. It is hard to say why people do anything they do, unless you are them.
Here is page 100 of What is It?, from around exactly a year ago, how many thread starters' usernames do you actually recognize, and how many have more than around 20 or 30 posts?
What Is It? - Page 100