Why are old posts from five years ago still being commented on?
When you look, there are posts for Musescore 3 still being commented on like two weeks ago. Can't these old posts be closed as their issues are resolved? You can still see them but you shouldn't be still able to comment on them. I think the comments should close after a while or upon resolution. And make new posts (not new comment replies) rise to the top. Or at least give us an option to see posts by date posted and comments by date posted, that way we can fine tune the Forum Topic section ourselves.
Comments
Why preventing it?
Usually the reason is that a user found those topics through a search when trying to fix their issue. After some years, new options might be available or alternative workarounds.
It's also important to keep in mind this is general discussion forum and like most similar general discussion forums, there is no concept of things being "issues" that can be "closed". We do have one of those too - the Issue tracker - and by default, searches don't include closed issues.
But, it's true that the search facility built into this site is pretty limited and not as useful as it could be. Using Google to search the site - prefaced with "site:musescore.org" - usually gives better results.