It's supposed to be arbitrary. Copyright is not some sort of human right or divine commandment that creators have. It's a legal device to incentivize a certain type of behavior. Like tax cuts and subsidies, governments can dangle it in front of people whenever it wants.

Monkeys and LLMs (so far) either don't understand or don't need such incentives, so they don't get copyright.

Humans dont need incentives either, we an abolish it.