The blanket rule is because even "good" necroposts tend to encourage bad ones. If I'm on an active forum and I revive some ancient discussion with a useful comment, lots of people will click on the thread and try to argue with the old posts.
The generally recommended alternative is to make a new thread and simply link to the old one so you don't confuse people.
The reason the term exists is because it's a blanket rule based on the age of the reply, not based on the intent or context.