It is cost free. It prevents people from filing it again. It provides insight into later problems. It provides a place where people can ask and offer advice on working around it. Sometimes an intractable bug becomes more tractable later. It can provide a clue to a later bug report. Sometimes someone new has a brainwave on how to fix it.
That would require a project be worthwhile to stick around long enough for an opportunity for someone new to come along, let alone come along and be interested in it. It's obvious that most career programmers are only used to ("interested in"?) pushing devops shovelware onto GitHub and other forms of busywork, so it shows in their bug-handling hygiene.
They are still problems that need fixing, some day someone may be doing work on the project without knowing what to work on and having a backlog of issues categorised by importance/etc will be useful in that situation.