Because they have family. 20 years, so you and your dependents can enjoy the fruits of your labour, even if you died of cancer, or old age, or you got hit by a bus 2 weeks after publishing your best work yet at 32. Don't have a will? Then sure, have the copyright expire. But if you have family, and they're in your will, they are most certainly entitled to those fruits until the copyright expires, just like how they can get any other business non-copyright-workers can set up. 20 years seems perfectly fair for that.

And if that means you "leave a legacy" for 20 years, that your family can cash out on: no different from someone leaving their company or their land titles to their kids. Except that company or land will stick around a lot longer.