Confirmation is a sacrament because it is [u:2zbua4jn]another[/u:2zbua4jn] time where we receive God’s grace when the Holy Spirit decends upon us. Thus, it is a kind of separate event from baptism. Of course there are similarities. Yes, we do profess our beliefs as we (or our parents) did at our baptism. At confirmation we become full members of the Church. It is because we receive the grace of God again that confirmation is a sacrament.
Jesus’ diciples were baptized early in his ministry, but it was a separate “event” of receiving grace when the Holy Spirit decended among them.