I think one issue, is that the world changes around an app or library. In these cases, the code base could need updating as a side effect of this change. Also,

I think one issue, is that the world changes around an app or library. In these cases, the code base could need updating as a side effect of this change. Also, not many people (especially me included) are able to write something that's perfect first time. In an ideal world, other developers would be testing and challenging what's been written; which would meant the product or library needs updating. I think it's (perhaps ideally) likely, that at least one of these factors is relevant.