The stories are copyrighted, but the characters inside are also trademarked. You can go distribute Steamboat Willie without consequences, but Mickey is still Disney IP.
And what stops you from making new content with Mickey Mouse in it is the set of copyrights Disney holds on Mickey Mouse. The trademark does nothing.
Established law says that every publication of a work involving a copyrighted character creates a new version of that character whose copyright extends for the full period starting from the publication of that work. This came up when someone wrote a story about Sherlock Holmes, who was out of copyright, and they were sued, successfully, on the theory that they had used aspects of Sherlock Holmes' personality that were developed in stories still under copyright.
