A very reasonable argument. However, pragmatically, the real reason is that the driving force behind copyright law in the United States (and therefore in much

A very reasonable argument. However, pragmatically, the real reason is that the driving force behind copyright law in the United States (and therefore in much of the world) is the Walt Disney Corporation, which has decided that it will use the full force of its political weight to ensure that no work featuring Mickey Mouse ever enters the public domain. This currently requires that the length of copyright be 70 years. However, since Steamboat Willy would under these terms become public domain in 2023, I would be willing to bet that we'll see some changes to copyright law around 2022. Disney has no such interest in patents and, as a result, no reason to lobby for the extension of their terms.