This tendency to close "stale" issues is a symptom of Goodhart's law [0]. Once the number of open issues is used as a proxy for software quality, then lowering that number becomes a goal in itself. And thus it ceases to be a pertinent metric, and it might even become counter-productive by discouraging contributors.
Another reason I think is that it's also a nice excuse to avoid working on old boring stuff and focus on new exciting stuff.