If you do a Google image search for [bible] and look at typical layouts used, I think this might be apparent.

Less obvious is that bibles are often printed on thin paper to fit a big document in a small space, so having things line up on one side of a leaf to the other is more important than in most other books.

I did that, but it wasn't apparent. I don't know much about typography, and less about why TeX can't manage.

One of the examples I found is https://www.behance.net/gallery/91186859/Typography-of-the-B... .

Is it to have parallel translations keep the verses side-by-side, as the first image shows between German and Greek?

I did mention the thin paper example earlier. :)