Why does Genesis say all nations would be blessed in Abraham's seed?
All nations are blessed in Abraham's seed because everything must come through this one little nation, Israel. Every word of Scripture came out of Israel, and unto them were committed the oracles of God. Abraham obeyed God's voice and kept His charge, commandments, statutes, and laws, and the Abrahamic Covenant promises blessing to all nations through that line.
Les explains that in this covenant passage God tells Isaac He will give all these countries, multiply his seed as the stars of heaven, and give the countries to his seed, adding, “in thy seed shall all the nations of the earth be blessed.” The obvious question is how God can speak of nations while dealing with Israel. Les answers that everything has to come through this one little nation; every word of the Book came out of Israel, and all the writers of Scripture were Jews. Paul makes this plain in Romans 3: what advantage has the Jew? Much in every way, chiefly that unto them were committed the oracles or the word of God. That is what makes this Book unique from any other religious book. The Abrahamic Covenant thus carries the promise that all nations will be blessed in Abraham's seed, and the context adds that it is because Abraham obeyed God's voice, kept His charge, commandments, statutes, and laws. The narrative then moves on to Jacob in chapter 46.
And I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries; and in thy seed shall all the nations of the earth be blessed;
Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.