Basically, because it's bad forum manners. The topic was exauhsted. Now people have to re-read the entire thread in order to figure out what the topic was even about. Some folks might even post replys to posts higher up in the thread not realizing how old those original posts were. It's a waste of time and rather annoying to find that you've accidentally written a 30 line response to a poster who hasn't been active on the forum for a year.
Breaking down bad habits,
and building new ones.