It's because, at least in the US, AI generated code cannot be protected by copyright.

<https://www.copyright.gov/ai/>

<https://www.copyright.gov/ai/Copyright-and-Artificial-Intell...>

  The Office concludes that, given current generally available technology, prompts alone do not provide sufficient human control to make users of an AI system the authors of the output.

What I was able to skim is a bit more complex like that. Page 42 of Part 2 says using it "as a tool" produces copyrightable works:

> As described above, in many circumstances these outputs will be copyrightable in whole or in part—where AI is used as a tool, and where a human has been able to determine the expressive elements they contain. Prompts alone, however, at this stage are unlikely to satisfy those requirements.

Then, since this stuff is changing a lot, they hedge with:

> The Office continues to monitor technological and legal developments to evaluate any need for a different approach.