That's much more true for the new testament than the old. The main four books (Matthew, Mark, Luke, and John) emerged in such a condensed time frame with such similar structure and content it's very difficult to reason about the genesis of them outside of comparisons to each other. This gets even more complicated when you realize that names are re-used and authors likely intentionally made use of pseudonyms, so it's very hard to draw conclusions of authorship from third-party sources. By the time historians start to mention it Christianity had been wildly popular for centuries.

However, much of the historiography dealing with authorship—including the research done by many distinct churches—absolutely treats them as the product of time and multiple contributors. I think there's a tendency to view the simplest narratives as representative because those are the narratives that tend to propagate the widest and fastest, but it's just not representative of what serious scholars think.

They aren't assuming that, and it's weird that you would claim they are. Where do you get this? Those in the field of textual criticism are well aware of the existence of "variant texts"—those "textual witnesses" (documents) that differ for various reasons from the sources they were copied from.