Probably part why I'm not a professional developer, but it's the principle of the thing

There's no 'principle' here, that's something that simply would not be possible in any sort of large project. To suggest all professional software developers read every line of every library before using it is ridiculously unworkable.

...but do yoz "understand libraries" by reading every line of their code, or by reading the documentation? And only in the parts you're actually interested in?

Libraries can be audited. LLM generated code cannot.

Edit: to clarify, it is impossible to audit all LLM generated code across a number of projects, that would replace a single library. It simply won’t happen, because there will always be a non trivial number of users who will copy and paste code without inspecting it. In contrast, widely used open source libraries may be audited by a small subset of their users, and the rest would benefit from that.