The idea is that -someone- reviewed the lines of code they modified, leading to the sum of code being looked at by many human eyes. Using someone else's code has always been a system of trust in this way.
AI/Vibecoding breaks this social trust, hence the general aversion to using those codebases or bringing LLM code in. It's not really about the general quality of the agents. It's about the code no longer being reviewed and understood by a human. How can one place trust in that? Unit tests don't replace careful review and considerate design.