The machine code generated by a compiler is deterministic and reproducible (assuming using same chain/tool versions), which is why reading the human readable pre-compiled source is sufficient.

Reading a prompt but not reading the non-deterministic/non-reproducible LLM output is not comparable.

How do you know it does what you want it to do without reading it? Tests? How do you know what they test? Yes, clicking a button in a browser and getting the result you want satisfies most, but that only works on the most basic systems. Once a code base grows large enough, any one agent reading in its context wont understand the whole, and if no human does either, it becomes unworkable.