Editing can substantially change an answer or question. These changes, if accepted, may increase the value of the answer/question and the number the up-votes it gets. These should be reflected in the score of the editor making these changes, not only of the initial creator.
Editors get credit (at least those with less than 2000 reputation) by getting 2 points of reputation for each valid edit. In the end the original content came from the user. You just helped them improve it, fix it, or clean it up. If you're adding mountains of new information to the answer and want reputation, why not just add a new answer?