Because it is impossible to read the Bible as a work of history.
People that don’t study history don’t really see how the discipline of history evolved over time. Paper was once really, really expensive, as were the services of scribes and archivists. The expense made it the pure domain of the state. It wasn’t until the 1700s when books became cheap enough for everyone to be able to have access to them.
Before books were common, they were typically locked up in libraries.
When this stuff is so expensive there’s just no way to turn the careful study and cataloguing and rigorous analysis of information, i.e. history, into a legitimate domain. Instead you have source materials of varying quality and written for various reasons, such as the king needing justification for a particular policy he wanted to enact. The scribe’s not just going to tell him no because it would be ahistorical.
The Bible and other scriptures of the day came about at a point in time where writing was in it’s barest infancy. It’s not just the technology that was brand new, also the societies that created religions were just then starting to claw their way out of subsistence agriculture, allowing the development of aristocracies.
Religion was sold by the aristocracy to the people, in a way precisely similar to how Lucasfilm makes Star Wars today. It was supposed to be better, more coherent, more interesting, than the old folk tales and beliefs. Organized religion has always been a construct, a work of art, fiction. People that study history and actually look at the old books and try to make sense of what they find can’t hide from that determination.
That all said, the Bible is considered to be a work of history. It’s exactly as historical as other books of the day are. Which is to say, not very.