For example, the author of this unanswered question no longer exist. Should they be deleted? (so I can flag them...)
Note I am just referring to unanswered questions for which the user has gone.
Stack Exchange is not a help desk. Questions and their answers are supposed to be helpful for (potentially) everybody, not just the one person asking them. Deleting a worthwhile question, even if unanswered, deprives future readers of the knowledge gained from the question itself. Sometimes asking the right question is the most difficult step towards a solution. Hopefully a few people have noticed the value of such a question and voted for it despite the lack of answers.
Maybe one day later someone comes along and answers the question and another solves their problem with the help of that answer. Or maybe someone encounters the exact same issue and puts a bounty on the question instead of writing their own.
If you look at the reasons why a question may be closed or deleted, you'll find that a non-responsive question author is not among them per se. Only when the answer to a question requires more information from its author and such information is unlikely to be obtained for whatever reason (OP is unresponsive, uncooperative, has serious language comprehension issues, their account vanished, etc.) that question should be closed.