• Copyright. A publisher typically holds the copyright on material in its magazines, conference proceedings, and such. It can't publish something to which someone else already holds copyright — publishing a significant piece of text without attributing it to the other publication is plagiarism and a copyright violation, even if the same person authored both works.
• Integrity. Even if content isn't word-for-word identical, if it imparts the same basic information, it should cite the earlier work. This is true whether the earlier work is published, has been accepted for publication, or is currently under review. General guidelines exist for how much new material must appear in a manuscript to be published as a separate entity, as I'll discuss later.
• Timeliness. Submitting papers simultaneously to multiple venues, then withdrawing them once a publication has accepted the submission, would let authors publish their papers a lot faster. However, this results in wasted effort and uncertainty for the other venues — especially if they're close to accepting a paper, only to have it withdrawn. Generally, submitting a paper to a conference or periodical is an agreement that if the reviewers accept it, the authors will edit it and submit the final version for publication. Withdrawing an accepted paper breaks all assumptions about what will be published and when.
• Wasted resources. Reviewers are a valuable resource — they volunteer their time to help the peer-review process succeed. Having a reviewer evaluate a manuscript that will never be published wastes that time. In contrast, if an author submits to some venue and is rejected, he or she can improve the manuscript and submit it elsewhere (that is, they haven't wasted any effort, except to the extent that they tailor a submission to one venue and not another).