Why is Christ called the "only begotten Son" of God, since we are also the children of God?

Because Christ alone is the eternal and natural Son of God; (a) but we are children adopted of God, by grace, for his sake. (b)

(a) John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God. John 1:2 The same was in the beginning with God. John 1:3 All things were made by him; and without him was not any thing made that was made. John 1:14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. John 1:18 No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him. Heb.1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, Heb.1:2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds; John 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. 1 John 4:9 In this was manifested the love of God toward us, because that God sent his only begotten Son into the world, that we might live through him. Rom.8:32 He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things? (b) Rom.8:15 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. Rom.8:16 The Spirit itself beareth witness with our spirit, that we are the children of God: Rom.8:17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together. John 1:12 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name: Gal.4:6 And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. Eph.1:5 Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will, Eph.1:6 To the praise of the glory of his grace, wherein he hath made us accepted in the beloved.