A Creator's Claim to Original Thought

There are two things I'm not sure about when it comes to copyrights. I'm a little confused about the topic of copyrighting.Why should the copyright date expire? First authors were outliving their copyright dates, and then the dates were changed to a certain date after the author's death. I don't think the date should expire at all. That's just making life complicated. The other thing I am confused about is why movies cannot be altered and resold. The Clean Flicks company got in trouble for altering movies and then reselling them. Books, however, can be altered and resold. What makes them so different?