For two primary reasons:IF we already are by nature children of God in the pre-mortal spirit existence, then why would anyone need the RIGHT TO BECOME God's children--if they already ARE his spirit children?
1) We are spirit children to God the Father--not Jesus Christ. The adoption is to Jesus Christ.
2) We are adopted in the flesh--which Jesus Christ is also not the natural father of. We have earthly parents pertaining to that.
Bonnie--you are aware Jesus Christ claimed to have the same God and Father as all men do, as to the spirit--right?
John 20:17---King James Version
17 Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.