Ah, but how do you know that there were these supernatural things called angels actually coming down to warn of these events, outside of the Bible itself which was written down hundreds of years after the fact, by supporters of the religion whose history it purports to describe? IOW, the warnings from the netherworld sound like self-serving post-hoc rationalizations.
And your assertion is based on...what? Many of the books - and in this case, especially the Book of Exodus - are claimed to have been "written down" by first-hand eyewitnesses to the events.
The core issue here isn't when they were written, but rather what is written. I don't know a soul who contests the mundane aspects of the Biblical accounts (city names, birth records, clothing style, etc) but supports the miraculous accounts. It's always the other way around. People are unwilling to accept the historical nature of the book because it carries with it an ethical and religious message as well. So they dispute the former, hoping to discredit the latter. Believing they've been successful, they think they and their behavior are no longer accountable to rules and persons they haven't - and don't - want to obey. That may not be your personal take on it, but it holds true in my personal experience. I would think the opposite of your opinion would be more believable - it would be entirely self-serving and more believable for the "losers" in said events, i.e. the Egyptians, Canaanites, etc., to re-write their own histories to make it appear they weren't quite so thoroughly humiliated by a God they refused to believe in or obey.
In modern politics, we would call that "spin control."