Abraham was not a random guy that God reached out to. Shem is Abrahams's great grandfather x 7 or 8. What moral law was Abraham operating from? Why do we see continuity in altars, sacrificial offerings, and calling on the name of the Lord? Genesis 26:5 declares "because Abraham obeyed My voice and kept My charge, My commandments, My statutes, and My laws." How could Abraham have done this with no Torah? We see through the narrative God's character, and we draw from what he does in contrast to what humans do.