The difference is that you're asking it to perform one intellectual task (write a program) instead of 100 menial tasks (parse a file). To the LLM the two are the same level of complexity, so performing less work means less possibility of error.
Also, the LLM is more likely to fail spectacularly by hallucinating APIs when writing a script, and more likely to fail subtly on parsing tasks.
In addition to what you say, it can also be easier for a (appropriately-skilled) human to verify a small program than to verify voluminous parsing output, plus, as you say, there's the semi-automated "verification" of a very-wrong program failing to execute.
Also, the LLM is more likely to fail spectacularly by hallucinating APIs when writing a script, and more likely to fail subtly on parsing tasks.