Because every time you run the LLM it will generate a new library, with new and surprising bugs.
It's better to take an existing, already curated and tested library. Which, yes, may have been generated by an LLM, but has been curated beyond the skill of the LLM.
The value of a library is not in the code it’s in the operations. It’s been curated and tested by multiple people in multiple environments. One of the ways to code at a high level is to delegate the cognitive load.
If you really can one shot it and it’s simple(left-pad). Great. But most things aren’t my that simple, the third time you have to think about it, it’s probably a net loss.
It's better to take an existing, already curated and tested library. Which, yes, may have been generated by an LLM, but has been curated beyond the skill of the LLM.