If copyrights disappeared after death, from a business standpoint it would become much harder to value a copyright when you also have to factor in the chance of the author dying and you immediately losing the related IP.
It preserves the “you own your work for your natural life”. Whether that’s what copyright should be is a separate question, and I think the answer is absolutely not, but easier to change gradually than all at once.
I agree X years after creation sounds like a much better system though.
Depending on the length of the term, this may not work so well for types of books that have a longer tail (e.g., sheet music), but the current system has drawbacks too.