Jack Shaffer has stern words on why writers plagiarize. Of his reasons, I think that "Ambition often Exceeds" Talent may have been part of not only Kaavya Viswanathan but also Ben Domenech (lest we forget Mr. Red State).
Jack also rightly says that writing is hard work--but also, if you want to be published, a writer has to get used to having all that hard work pulled apart by an editor. If a writer is precious about his/her work, the editor's pen will indeed be mightier than the sword.
When the editor's pen is too much, present the editor with something that will make him/her just love you to pieces!
Plagiarism probably comes from someplace where there's a mixture of huge ego, inordinate drive, and flagging self-esteem....stuff that no amount of cheating or stealing that getting published could ever fix.