Probably because most are at work or doing something else. Sometimes if a computer or device used is on, SAS is in the background in the browser, they are logged in, it might show up on that online list.
Sometimes it might not need them to open the browser, could be just still logged in, computer or device is on.
I think it might be the above, accused of spamming if post too much, don't want a certain party talking to you or just online anxiety / fear or not sure how to talk to others.
Maybe you can create a talk to others forum or thread, just for random conversations about any subject? Much like the dot thread.
Going by the vent only thread the mods created, I think that would answer your question as to most just want to vent, not be answered to. It defeats the purpose of a support website in some ways.
Other forums in UK where have seen some very intelligent conversations and are a joy to read. Like reading a good novel. Also some stuff passes, where it doesn't here unless in 18+ forum.
Some forums use restricted access based on gender (so can talk about male or female issues). They are really active and thread's top out at around 130 post's really quickly sometimes (post limited so more threads are created and can't edit post's which reduces load on forum server as editing post's creates many copies which forum software has to load).
They are not moderated much at all. Men talk about women, women talk about women issues, men without offending each other (removing the need for the gender rule here).
If look at previous thread's on here, mainly men talking about women or vice versa which made the forum more active.
Sometimes creating too strict rules can stop people posting which has been mentioned many times over. There are other ways to deal with the same issues as mentioned above and requires less intervention by mods, less stress on mods dealing with what they perceive to be difficult people.