But until the 1976 US Berne agreement, you did have to formally register your copyright with the copyright office. In the 19th century (US), you actually had to publish the item for the copyright to be accepted as valid.
Every other form of I.P requires proper registration, every piece of real property requires formal registration, and there seems to be no problem handling the ownership of said properties. Once again, I ask, what makes copyright so special. . .