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

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 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.     

https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fgjq7rzqjeb2b1.png

DeKay