If copyright doesn't begin until exhibition, then the outright theft of private works is legalized. [1] You'll have reintroduced the problems that implicit copyright existing at the moment of creation was intended to solve.
So to fix that, we can then either recognize the right of the author prior to publishing/exhibition with more complicated rules, or simply extend the term a little and maintain simplicity.
[1] Joe writes a book. He shows it to Bob for notes. Bob publishes it under his own name. The copyright would belong to Bob.
There is a sane middle ground. Copyright is granted upon creation, but infringement can only be pursued in cases where theft (or strongly probably) theft can be shown. Someone you don't know has the same idea? Tough cookies, you should have published. After public unveiling (registration, publication, etc), the timer starts. This puts some onus on the creator to not spread his work around willy-nilly, but allows him to shop publishers etc.
So to fix that, we can then either recognize the right of the author prior to publishing/exhibition with more complicated rules, or simply extend the term a little and maintain simplicity.
[1] Joe writes a book. He shows it to Bob for notes. Bob publishes it under his own name. The copyright would belong to Bob.