Besides the reasons people mentioned here, necrobumping is generally bad because it takes something that has since expired to time and makes it appear fresh.
What I mean with this is that, whenever someone necrobumps a thread, a lot of people do not really realize it was necrobumped and often come to the wrong conclusion that all the previous posts in the thread are themselves as recent as the latest one, because that's usually what happens in a normal thread.
This means that even though most people in that topic may not even be active anymore, people doing the posting won't realize that and assume they're active people right now.
They will also assume that whichever info there is up to date, although that is likely to not be the case anymore.
And those who actually pay attention, realize the necrobump itself and cannot help but refer to that exact fact and quickly lose interest in the thread altogether, along with everyone else who were "fooled" by the fake freshness of the content.
Furthermore, people aren't the only ones fooled by this: search engines crawling the forum will think the pages are recent as well, and give then unwarranted relevance in search results.
If this happens once in a while it's not the end of the world, but we've had here members that will suddenly go on a necrobumping spree for some reason, and all of a sudden we get half of the front page with threads that may be up to a decade old already (or more), which is why it is usually stopped as soon as it starts.
As for why there isn't a message or something, or some other automated mechanism to deal with this, one thing everyone needs to remember is that this forum is paid for regularly, so it remains online for everyone, but maintenance is not something that is usually done very often, especially when it comes to add new features or setting up configurations.
If you want to have something done in this regard, just create a thread in the
Suggestions, Criticism, Feedback section about it, and maybe something might actually get done later on.
This is exactly how the
double posting issue was addressed.