SrtaA Necros aren't cool for a variety of reasons
1) The majority of people who are in a thread that is necro'd are inactive. This makes the discussion stale/has people replying to things that other people wrote months ago when these people are likely/possibly not even online anymore.
2) Plenty of these necro'd threads deal with irrelevant topics, like policies WN has already made changes to since the time that the thread has been posted (e.g. stuff about turning farming mode on/off, a thing that WN has since disabled). And most necro-ing replies are, frankly, irrelevant as well. No one really cares about someone's contribution to a conversation from a year ago, but if a new thread was made with their argument, it would be much more relevant. It's overall just not very productive.
3) A lot of the threads that are necro'd are from people who are just asking basic questions-- like this thread, for example. Or, a more common example is someone going on say, a thread ranting about how much the original poster hates fast passes, then asking about how fast fast passes expire. Their question would be more accessible to the community if they made their own thread and asked the question, due to the fact that if people came across a thread titled "I hate WN's new system!", they would probably not come across the question, but if they came across a thread titled "How fast do fast passes expire?", they would feel more inclined to answer the question with an answer.
4) It's general forum etiquette to not necro. A lot of these forums themselves have systems put in place to archive/lock old threads, but WN's forum is sadly limited.
And lastly, if a thread that's genuinely productive/creative gets necro'd (like Clowniac's describe your story in the worst way thread), then this thread won't get locked. Or those really cool threads where it's like counting-competition-- they won't get locked either-- because those threads add on to the more fun environment of the forum (I always think that we should have more balabala threads instead of all these report bugs/question threads) and I like to encourage that kind of necro-activity over angry people or people who don't read FAQs (no shade lmao).