No one said you cannot run LLMs with the same task more than once. For my local tooling, I usually use the process of "Do X with previously accumulated results, add new results if they come up, otherwise reply with just Y" and then you put that into a loop until LLM signals it's done. Software-wise, you could add so it continues beyond that too, for extra assurance.

In general for chat platforms you're right though, uploading/copy-pasting long documents and asking the LLM to find not one, but multiple needles in a haystack tend to give you really poor results. You need a workflow/process for getting accuracy for those sort of tasks.