The automatic state for content is "protected by copyright," unless a different arrangement is made.

The automatic state for contract terms is "not agreed to," unless a different arrangement is made.

tl;dr You need proof (written or otherwise) that a contact happened; not so with copyright protection.

Same goes for copyright notices. For example, the notice makes it hard for a violator to use a negligence defense, effectively increasing the penalty for violations.

It's a bit like putting a "stealing this car is illegal"-sticker on your car - sure, you made that clear, but "I didn't know stealing this car was illegal" is not going to fly as a defense either way.

But proof is not relevant for a copyright. Even without writing, the existence of the copyright is not in doubt.