“Just consider the example of a published halachic work. There are financial and time (to write, edit, publish and distribute) constraints. It’s just not feasible for an author to think of let alone cover every imaginable custom and exception.
Even were such a thing possible, the resulting tome would be so huge that just searching for a specific ruling would be a daunting and time consuming task.”
I think it’s more than that. When a Posek answers a shailoh, he’s not answering a shailoh, rather he is answering a person. It’s not like he’s a computer that takes in all the relevant factors and spits out the correct halacha. He has to know what kind of answer would be appropriate for that particular person. Since every person is unique, there are literally an infinite number of shailohs that a Rov may encounter.