Some people actively participate but rarely if ever start a thread.

When I really ramped up this hobby in 2019 or so, I started many threads, most of them trying to get at questions for which easy information would be hard to find -- for example, "Is there something special about low efficiency speakers and higher powered amplifiers?" or, "My outlets are reversed in my listening space -- is that dangerous for equipment." Etc. The discussions which have ensued from many of my threads have been so informed that I’ve saved many of them as PDFs into a library of files on various topics in audio.

I do a fair amount of diy/modding, my experiences not applicable or of interest to most. Also, after many decades in this hobby my curiosity has waned. Between gathered knowledge and direct experience I've answered nearly all the questions I ever asked.

For me, same reason I like editing something that already exists than to start with a blank page.... 

Some members may be more active on other forums or facebook, i suppose

I primarily come here to check classifieds.

Some are happy enough with their system(s) as-is, no longer searching or less so, yet don't mind trying to help others when they can, is another reason perhaps.