For the same reason that once a piece of legislation is passed, you're still allowed to comment on it and/or vote for its repeal. Just because a question is closed by a small group of individuals, myself included, doesn't mean the issue is suddenly null and void.
Super User (as well as the remainder of the Stack Exchange group of websites) is not a discussion forum. Our job as moderators is to provide the best quality information, and that includes questions that were deemed as worthy of being closed. As opposed to a discussion forum, which are often run by a select group of administrators, there is no "end-all, say-all" users here; we're a community. We respect everyone's opinion, both before and after actions have been made.
Finally, moderators can also vote to re-open questions if they are improved in quality or scope. It's much better to reduce the number of questions on the site and improve their quality, rather than continually re-posting better higher, quality questions covering mostly the same scope.