Because after a period new information comes up, or the way people think may change, or move on and the old discussions aren't relevant enough. On the bright side you can re-post a topic if it's been dead for more than a month instead of bumping old one. You can quote important posts or previously discussed conclusion and incorporate your present outlook to it and restart the discussion.What's so bad about bumping threads?
If it's a good thread why not bring it back?