Because the whole point of copyrights is to promote creativity. No one at Disney today had anything to do with creating Mickey Mouse. Why should they have the exclusive right to create derivative works? The people who are there seem perfectly capable of coming up with new characters. And the copyright expiring will push them to do so.

And besides, it’s not really about Mickey Mouse. It’s also about art, music, and literature. Beethoven’s heirs shouldn’t be getting a check every time an orchestra plays the 5th Symphony.