In my experience, the biggest problem is that maintainable code necessarily requires extending/adapting existing structures rather than just slapping a feature onto the side.

And if we're not just talking boilerplate, then this necessarily requires understanding the existing logic, which problems it solves, and how you can mold it to continue to solve those problems, while also solving the new problem.

For that, you can't just review the code afterwards. You have to do the understanding yourself.
And once you have a clear understanding, it's likely that the actual code change is rather trivial. At least more trivial than trying to convey your precise understanding to an LLM/intern/etc..