Why do people who create music, books and films feel entitled to earn more then their fans would over multiple lifetimes?
Why do hotels and apartments charge rent for rooms over multiple lifetimes? Why do farmers make profit off the same land for multiple generations? The answer to all these questions is: the assets provide value to customers over multiple generations. That's why.
Originally copyright protection was for X years then it became public domain. THen it was X+10, X+50, etc.... Sometimes 'retroactive' protecting work whcih should have been public domain.
Originally copyright was owned by the authors of books. And since authors were ordinary working class folks, the lawmakers probably wrote copyright laws to limit the amount of wealth to match that of a working class man. But as time passed, publishers began to own the copyright of books, so the lawmakers rewrote laws to be more suitable for ruling class (capitalists).
There's absolutely no reason copyright should not be infinite years, other than the limit the amount of wealth to copyright holders. The limited length of copyright law is a scam to deprive the creative class of people of their richly deserved wealth, and you are an even bigger scammer to want to further reduce the copyright years.