I think the key distinction is that it's difficult to determine whether a single commit or pull request is bad code at a glance, but it's easy to determine when the codebase as a whole has gone bad due to many low quality commits.

The latter often doesn't even require looking at the code, you can usually feel it just by using the software. From my experience, all software primarily written by AI is full of little bugs and inconsistencies that reflect bad code architecture (such as two very similar pieces of functionality in two different places behaving in wildly different ways, due to the AI being unaware of the first when asked to implement the second and writing the code twice)