Well, it notifies people who have followed the post after the discussion has long dropped, and a new comment would be irrelevant and annoying to those who have followed a post sends unnecessary/annoying pings to a user's notification box.

However, there is no explicit rule banning necroposting (as far as I can see in the wiki guidelines, I might've missed something).

TLDR:

It's just not very pleasant to suddenly receive a notification on a post you upvoted like 2 months ago and forgot about. Kind of annoying.

Just want to point out that you can always opt to unfollow a post with a simple button click. Personally, I've never seen a problem with necro-posting. People join Wikis at different point in time. Why should they have to miss out on a conversation/post they take an interest in just because they had the misfortune of joining past the necro-post date. Or even people who are already members of the Wiki, some of them take breaks, or some of them miss a post and later check out older ones and stumble across them later. Why should they have to miss out because a post is considered too old. If you get a notification for an old post and you're not interested in follow it anymore, then just unfollow it (I do it all the time for posts I don't want to follow anymore) and then everyone can be happy, no problem.