As a newbie, I'm much less inclined to altruistic considerations like what I can add to a forum. I'm much more interested in what I can get from the forum. If the forum has useful information, funny chatter, then users will share something one way or another.
You make a very valid point. People are attracted to what they can benefit from; it could be some social app like Facebook or Tiktok where they can scroll around and watch videos if they are board, or it could be Quora where they can ask questions about pretty much any topic and so on.
This is the same with Bitcointalk; ideally users who register an account here has some slight interest in learning about Bitcoin or crypto. It's also fine if along with that they learnt about signature campaigns and build an interest in it. But some users have no interest in Bitcoin or the forum at all, they just want to make a quick buck.
They may have learnt from a friend that there is a forum where you can join bounties and get rich like that. The ask for the URL, register and account, run straight to the bounty sections and start spamming sit no consideration to how the forum works. Until they need to earn one merit to rank up, then they start with the merit leaning posts in the hope a user will consider it.
The forum definitely has useful information and some funny chatter, but you have to be looking for it to find it or for it to be beneficial to you.