Trust is the most important part. You trust someone they made something themselves. They digitally sign their work with a public key that is known to be theirs. You can now verify they (the person you trust) made it.
Once the trusted creator's key is leaked, they are no longer trusted for future works.
AI made content can be freely signed as well, but if you don't trust the origin, the signature doesn't matter anyway since it will just verify it is coming from the AI creator.
The key thing is trust, the signature is just there to verify.

Throw the technical bit away. Just think of it as a signature. Yes, an old school, written-with-hand signature.

Does the bank trust me giving you $100 by you having this cheque? Yes. Why? Because I told them what my name is and what my signature is like.

Will the bank give you $100 if you stole my cheque and sign your name on it?