I would draw the difference here that a library is used by hundreds (of thousands) of people, and established across different scenarios. I don't read the boto3 library AWS provides, but I can trust them and the amount of customers enough to be certain enough that it behaves the way I expect it to. The same can't be said with code we write in silos at our workplace or at home. It simply does not have the same test bench.

Yes, libraries aren't bug-free, but they give me a reliable abstraction tested in the field. Not rarely you dig into library code if you notice unexpected behavior.

If we could rely on our LLM or colleague written code, or own code, have run through the same amount of requests, sure I wouldn't need to review it, as my confidence can be north of 99.9999% it works correctly. But we can't.