To answer your question we must discuss memes, and I don't exclusively mean funny internet images. Memes are units of culture, the equivalent of genes in biology. All topics on this forum are memes, all images, all words. Memes, like genes, are replicators in the sense that they self-propagate typically with change over time. Now memes are interesting creatures. They are successful only to the extent that they are advantageous to themselves, otherwise a meme is contributing to the survival of another meme.
To apply this to the forum, imagine a user posts a spam topic. If the thread invites more spam posting, it will be successful. If the thread is mindless and low effort, and the replies which follow are also mindless and low effort, it implies contributing to the thread is easy and not time consuming. Thus users will be more likely to propagate the meme because it is a low-effort reward. Take for instance this very post, or the one I made preceding it in the day discussion topic about "rules" (see my recent posts). Both are rather lengthy for a forum post, and have some thought put into them. In fact after writing the first paragraph of this post I paused for 30 minutes because my roommates entered and I had to do homework to look productive. I hypothesize this post will receive at most 5 replies, whereas the next spam topic will reach page two. Am I salty? Nah. I get a kick out of brown townyzing this stuff.