Well, that's because the NT clearly states the gentiles are now a part of the commonwealth of Isarel and fellow heirs with the Jews in Christ
Ephesians 3:6 This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.
Ephesians 2:11-16 Therefore remember that at one time you Gentiles in the flesh, called “the uncircumcision” by what is called the circumcision, which is made in the flesh by hands—
12remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world.
13But now in Christ Jesus you who once were far off have been brought near by the blood of Christ.
14For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility
15by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility.
Galatians 3:28-29 There is neither Jew nor Greek, there is neither slave
g nor free, there is no male and female, for you are all one in Christ Jesus. And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
I absolutely agree there are many OT promises that were spoke to the physical descendants of Abraham and fulfilled, according to the OT, with the physical descendants of Abraham (exodus 1, Joshua 21:45, Deuteronomy 1:10).
However, we do need to be careful to follow the NT on fulfillments of OT prophecies as well.
for example, according to Paul, how was the uniting of the northern kingdom (ephraim) and the southern kingdom (Judah) fulfilled? By the uniting of believing Jews and believing Gentiles in Christ. (Romans 9:24-26).