If we were to update the list automatically the page would be a total mess on a busy forum as topics would have to move around every time a new post comes in.
You do have a point there.
However, you could still do it in a Facebook-esque style:
When a reply is made to an existing topic (or a status, on FB) is made, do nothing (or alternatively, only update the reply count & "X posted Y minutes ago" snippet)
When a new topic is made, add it to the top of the list
List every topic in order of most recent reply (current behaviour)