The Supreme Court of the United States seems to think people create is because, in part, because of the possibility of extensions to existing copyright terms. That is, "I would not write this song if the copyright term were onnly my life plus 70 years, as the existing term states, but only if there is the possibility that the term is increased, someday."
Or, in their words:
The CTEA may also provide greater incentive for American and other authors to create and disseminate their work in the United States. ... (the United States could not play a leadership role in the give-and-take evolution of the international copyright system, indeed it would lose all flexibility, if the only way to promote the progress of science were to provide incentives to create new works).[Note that the justification here is taken from a law review paper by Shira Perlmutter, a vice president of AOL Time Warner.]
Now Playing: One More Day, One More Night - Tom Petty (Echo)