Why (and I realize this is one of those questions that sounds snide, but I do not mean it so) should a creator's kids gain from a piece of art or work of music after a creator's death?

Editing to clarify: I fully accept that copyright is a good thing to give incentives to people to make art and music and creative works, I just don't understand why that incentive should be transmissible to their kids, i.e. people who were presumably not involved in making it in the first place.