That is a perfectly valid opinion and POV to have. While I don't share it personally, I can totally understand why one has it. That does not change, that I personally actually do not care the slightest about the question of who or what wrote a novel. But that is just me, having studied literatur and fallen into a school of literary studies that held Roland Barthes' claim of the death of the author [1] in high regard.…
I think this might be the most pernicious strain of anti-intellectualism online: the idea that there is no subtext, that a text only communicates exactly what it lays out on the surface level. That's not at all what Barthes was saying; you've misunderstood him.
Writing is not a noiseless transmission of thoughts, and thoughts themselves are not totally uninfluenced by the outside world, and people do not always state exactly what they mean in the simplest possible words, and words are understood by the reader in ways that writers did not intend. At every stage, there is the possibility of subtext and context being present.
Your teachers weren't trying to get you to fix on the one perfect interpretation, because that idea (Barthes would agree) doesn't really make sense. Instead, they wanted you to interpret, explain, and justify in a complex and ambiguous space.