Why are most early Biblical books found in Sumerian texts from 2000 years before Moses wrote the Torah?

OK, this is my opinion only. My belief is that the information in the Bible is ancient and was known from the beginning. I doubt the time spans given in Genesis are correct, but the sequence and occurrences are. Genesis is attributed to Moses but the knowledge is much more ancient. It is NOT that Genesis stole the stories from other cultures, but instead that all cultures sprang from a common source.