Because most of the threads started by intelligent reasonable people, capable of understanding an argument and either coming to an agreement, or agreeing to disagree without antagonism end up with everyone agreeing, or agreeing to disagree, usually in a calm polite manner, after a few pages.
The threads started by bat-guano insane idiots live on because the bat-guano insane idiots refuse to alter their stance one iota, regardless of what evidence is produced to prove that they are bat-guano insane idiots, and continue to argue their point endlessly, in the face of any evidence to the contrary, and most people here refuse to let the bat-guano insane idiots have the last word.
I think.