It’s the same with the promises that God made to Abraham and to his seed. Scripture doesn’t say, “and to your seeds.” That would mean many people. It says, “and to your seed.” That means one person—Christ.
People swear an oath by someone who’s greater than they are, and the oath guarantees that they’ll keep their promise. After that, nobody argues about whether the person really meant what they were saying.
So he had to be made like them—fully human in every way. That way he could serve God as a merciful and faithful high priest and purify the people from their sins.