They certainly could have bee written again 800 years after Moses, however, they were first written down by Moses himself. Even Jesus mentions that Moses wrote down his words.The stories were written down 800 years after the death of Moses. There was no Ur of the Chaldees. It would have been under water until the river changed course.
John 5:45-47
45 Do not think that I will accuse you to the Father: there is one that accuseth you, even Moses, in whom ye trust.
46 For had ye believed Moses, ye would have believed me: for he wrote of me.
47 But if ye believe not his writings, how shall ye believe my words?