Genesis 12 was promised to Abraham himself, not the Jews.
Genesis 12:2-3 "And I will make of THEE a great nation, and I will bless THEE, and make THY name great; and THOU shalt be a blessing: And I will bless them that bless THEE, and curse him that curseth THEE: and in THEE shall all families of the earth be blessed."
Thee, Thou, Thy are all singular forms of the second person pronoun, meaning that God was making a personal promise to Abraham himself; the promise does not fall on the people of Israel, because if it did, the scripture would say "You, ye, your" instead, which is the plural form of the second person pronouns.
So, do not misinterpret scripture - unbelieving Israel is not God's chosen people. Galatians 3:7 says that Christians are the children of Abraham, not the physical Jews.
Romans 2:28-29 "For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God."