So far as I can tell, the idea behind extending copyright two generations after the author's death was so that they could leave the rights to their children and grandchildren, and this would keep old or terminally ill authors motivated.

I mean, it's fun. Ever listened to the KLF, and things from the era before sampling was heavily sat on, such as the album 1987 (What the Fuck Is Going On?) - ? I don't claim it's very good, but it was definitely fun. And the motivation for using existing works, instead of creating your own, is similar to the motivation for using existing words, instead of creating your own. They're reference points, people recognize them, you can communicate with them instead of having to extract patience from the audience like they have to learn a new language for each work. And of course in practice the rules are fuzzy, so everybody sails close to the wind by imitating others and in this way we share a culture. Stealing their work is just sharing the culture more closely.