Because - per the pattern of behavior we're discussing in this thread - some developers close old issues, whether they were actually resolved or not, and it's becoming a common practice, particularly with the appearance of bots performing this function automatically.
Also, not everyone is using the most recent release.
Some issues are not actually bugs but user errors (e.g. because they haven't read the manual). Personally I'm not sure if it's better to close such issues or keep them open with a special label.
I do, because I realized people close stale issues just because (and then there are bots that do it now too). I'm happy I can still do it, and I worry that one day someone will have a bright idea to "archive" closed issues to where they cannot be easily found.