The job of a software developer is not just to prove that the software "works". The definition of "works" itself is often fuzzily defined and difficult to prove.

That is part of it, yes, but there are many others, such as ensuring that the new code is easy to understand and maintain by humans, makes the right tradeoffs, is reasonably efficient and secure, doesn't introduce a lot of technical debt, and so on.

These are things that LLMs often don't get right, and junior engineers need guidance with and mentoring from more experienced engineers to properly learn. Otherwise software that "works" today, will be much more difficult to make "work" tomorrow.