Because the Bible is typically printed on tissue-thin pages on special printing presses with tiny letters, and most people would never buy a novel printed that way.
People don’t buy Bibles to read them, they buy Bibles to show off to other people who buy Bibles. At most they’ll read a handful of verses over and over. You show me a person who’s read the Bible from cover to cover and I’ll give you even odds that person came out of the experience an atheist.
I’d warrant 90% of the Bibles sold in the United States never get read cover to cover. Nobody who prints Bibles expects their buyers to read the entire thing. On the other hand, people who publish novels do expect their buyers to read the whole thing.
(I’m serious about the “special printing presses,” by the way. Bibles are generally printed by specialized book printing companies that only print Bibles. The super-thin paper, typically linen mixed with highly chemically processed wood pulp, is called “scritta” and requires dedicated handling. Only certain types of presses can print on scritta, and only a handful of paper companies manufacture it.)