Many christians don't take the old testament literally...?

they will openly acknowledge that the stories were probably made up in order to teach a certain morality. my question why would this have been so difficult to do with the new testament as well? the whole topic of fulfilled prophecies could easily be explained simply by understanding that the authors of the new testament already had a knowledge of the old testament, as i'm sure most literate people did at that time. so couldn't they have just written stories to agree with old testament prophecies even if such events never took place?