Ask a publisher what length a novel should be and the answer will land within a fairly narrow range. That convergence is driven by production and pricing more than by narrative need.
Books are printed in fixed blocks of pages
A printed book is assembled from folded sheets, each producing a fixed number of pages. A manuscript that overruns a block by a few pages forces an additional one into the binding.
That extra block costs paper, printing and weight for every copy manufactured, and weight determines shipping cost.
Length therefore moves in steps rather than smoothly, and the difference between a manageable book and an expensive one can be a single chapter.
Price is set before the book is written
Publishers decide roughly what a novel will retail for early in the process, based on the author, the category and comparable titles.
Once the price is fixed, the manufacturing cost has to fit beneath it while leaving margin for the retailer, the distributor, the author and the publisher.
A very long novel by an unproven author breaks that arithmetic, which is why debut manuscripts are held to tighter limits than established names ever are.
Editors are also estimating attention
A long book asks for a larger commitment from a reader who has no reason yet to trust the author. Abandoned books generate poor word of mouth and few repeat purchases.
Established authors escape this because their readers have already decided, which is why successful novelists' books tend to grow longer over a career.
The editorial pressure to cut weakens at exactly the point where an author's sales make the extra printing cost easy to absorb.
Different categories sit at different lengths
Category romance runs short because it is read quickly and published frequently. Epic fantasy and science fiction run long because their readers expect immersion and the world takes space to build.
Thrillers sit in between, favouring pace over accumulation, and literary fiction varies most widely because its economics are less standardised.
These are conventions of each market rather than rules, but a manuscript that ignores its category's convention will be questioned before it is read.
Digital removed the constraint without removing the habit
An ebook has no page blocks and no shipping weight, so the manufacturing argument disappears entirely in digital-first publishing.
Lengths there have drifted, particularly among independent authors, but the print conventions persist wherever a physical edition is still planned.