I never understood it either. I mean, as a religion.
What is even sillier, is that as far as web forums are concerned, you have 50% of them which will go mad if you "necropost", and the other 50% which will go mad if you open a new thread when there is already an existing thread on the same topic (even buried), because these have the exact opposite written or unwritten rule. The latter ones love their 800 pages long topics; the former ones forbid you to follow up with a discussion even when it is the exact same topic, if the discussion stopped for a while.