It secures the benefits of copyright for work being produced by the creator up to death, for their heirs and dependents, which is why they were creating for money in the first place. People don’t just die after twenty years of resting on their laurels; everyone is creating copyrighted work. It’s a key part of the incentive to create lasting works of value.

One can make the case that this period should be more limited, or that the combination should be capped, but life+X is the right formulation, I think.

If IP rights end at death, there are some significant perverse incentives for offing big-name artists and authors and whatnot.

That dead man took the risk of not earning much in his lifetime, to continue feeding his family even after his passing. You might as well ask why does someone acquire life insurance.