Why is it against rules to necro old threads?

Isnt it basically: "We dont want you to talk about it anymore."?

Nope.

It's more that the game state changes, and we'd rather you make new content that people can dive right into discussion on rather than reviving a thread 6-months old with pre-existing conversation, some of which may no longer be at all relevant to the current situation.

The point is more: If it's become relevant again (note that it's not a necro if you respond to a still active thread), you might as well make a new thread.

Otherwise people who do not pay attention to when posts were made are bound to respond to extremely old posts, and some people might not want notifications for things they wrote a year ago. I suppose ironically that's not much of a problem anymore currently.

The real question is why are people looking at threads from months to years ago? It's just as easy, if not faster to create a brand new discussion.

Honestly, there is no valid reason for necros to be against the rules for necroing. If the boards dont want necros to be a thing, they need to implement an auto close feature for threads without responses for 30 days+. That should be back end, not user end. There is no legitimate reason to punish the user for something that can be solved with a proper board interface. If the boards designers dont want to setup the boards properly, its not the end users job to make sure they dont make a mistake when they clearly believe it to be of relevance.

Title answer, because relevancy, changes, and how much was already there.

As an example, a post asking for self mute from 3 years ago is no longer relevant because it exists now. If you comment on what was once a "hot" topic, some people really don't want to sort through 50+ comments to make sure a response to you is relevant now.

If they didn't want people to talk about it, they would simply delete the entire threads and threads related to it; and maybe tack a ban on just to emphasize. Instead, they typically delete the comment that necroed and gently suggest to create ones own post to encourage that one gets meaningful feedback.

Also, thanks for all those downvoted I got just for asking serious question.

LoL is a constantly changing game and some old topics wouldn't be relevant anymore. Most forums autolocks or prevents upvoting/downvoting on old topics , for example Gamefaqs and Reddit. Not sure why threads here don't autolock within one month of no bumps. Some people don't want to be reminded of their old threads or comments. Yes, there has been threads that got necro'd that I would want to comment on, but I rather keep the thread dead instead of bumping it again. People want to see new threads.