Why do copyrights last so long?

I strongly recommend that interested parties read this full article on Wikipedia covering copyright law and its evolution: History of copyright law

The good old House of Lords in England made the call in the mid 1700's that both Public Domain and statue-defined Copyright existed. France and the US were also grappling with how to promote, protect and ultimately share with the public any works. Remember, before this only a specific publisher in London would forever be the one with the right to print Shakespeare; a right under attack by devilish Scottish printers (how dare they!)

Now that the terms were subject to legislation, a moderate term of 14 years (with an extension of 14 more possible if the author was still alive) secured a limited monopoly that was intended to hold the author through to the end of his days and secure a reasonable inheritance for one's children. Not at all unexpected in societies that had Land Owners who derived profit by renting their land to use by farmers, workers or industry. (Read some Adam Smith if you want to hear more about how Rent is a fundamental element in economic understanding at the time.)

What is meant by copyright? 

Are eBooks protected bycopyright?

A hundred years later and we have internationalization of the scheme being defined in treaty. In the Berne Convention, signatories were all agreeing to Lifetime-plus-fifty-years as the new minimum term.

How did we get that jump? Again, the publishers constantly push things back toward their original conception of perpetual copyright. These expansions always cite the author's need, but benefit the business to an equal or greater amount. And terms have been pushed farther and farther out over the decades. Partially because it turns out there is big business in making sequels and derivative works.

Literature has turned from being primarily a means of disseminating information and turned into an entertainment venue where the public is voracious for new-but-familiar content. (Amusingly, this does not hold true for music, which falls completely out of public favor in about 40-50 years. Yet the RIAA is one of the fiercest fighters of copyright protections.)

Content publishers now fear that someone will make a better version of James Bond's Casino Royale if they are given the chance to make their own derivatives. And they are probably right. But that's not the best justification for 110-year-long copyright terms just so we can protect Mickey Mouse's first movie.