Because that's what the US Constitution requires [1]:
"To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries"
(Emphasis added)
Copyright is limited. It's just that the Mickey Mouse collective is constantly getting laws passed that push back how far away "limited" is. It's more like a pay-as-you-go scheme for unlimited copyright while skirting around the US constitution. And of course, because Disney has enough money they can also convince Congress to apply the law changes retroactively.
The content Mickey Mouse has been in is copyrighted. Mickey Mouse himself is trademarked. So you /may/ be able to reuse Steamboat Willie for whatever purposes you wish (assuming Disney Legal doesn't get Copyright extended), but you'll probably never get Mickey Mouse as a symbol to work with.
"Mickey Mouse collective" is a term I use for the pro-copyright-extension lobby. But yes, you're right that Mickey Mouse will probably never be a usable character.
I take a boycott from buying Disney products (including movie tickets / movies). I will gladly watch their movies if a friend has a copy (copyright law still hasn't taken that freedom away from me), but if no-one has a copy the movie couldn't have been that good anyway. :P