With long copyright terms, it encourages copyright holders to milk a single work for the length of the copyright (90+ years) and therefore discourages the creation of something new. It also encourages people to obtain copyrights to leverage them for profit, rather than making anything at all. A child of an artist can spend their entire life supported by their parent's copyright, and never has to make anything unique for as long as they live.
How is any of this good for creativity?