The promises were given to Abraham. They were also given to his seed. Scripture does not say, “and to seeds.” That means many people. It says, “and to your seed.” (Genesis 12:7; 13:15; 24:7) That means one person. And that one person is Christ.
Our parents trained us for a little while. They did what they thought was best. But God trains us for our good. He does this so we may share in his holiness.
So he had to be made like people, fully human in every way. Then he could serve God as a kind and faithful high priest. And then he could pay for the sins of the people by dying for them.