Where is it written that it must be written? Where's it written that only what's written is true? I'll stop here before getting too silly, but writing is only an expression of a preconceived thought.
I agree with your second sentence. What we decide is moral is not necessarily so. Morality transcends us. We can discover it, not create it.
It's a shame God gave us morality in the form of and endless puzzle we're tasked with solving. Also a shame how much suffering he'd allow in our pursuit of its solution. But then again the show wouldn't be so entertaining would it? And that's the end goal, a good story.