TeX usually tries to adjust the space between lines so that a single typeblock is more visually appealing. Unfortunately, doing this adjustment independently for side-by-side columns leads to output where the text lines don't line up, and that is way less visually appealing.
Why is that particularly relevant to typesetting a Bible?
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.
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. :)