The reason that Jesus was born as a baby and not made as a mature 30-year-old man is that in order for him to be the Messiah and fulfill prophecy, He needed to be born under the law as Galatians 4:4 states He was. Part of the requirement of the law was to be circumcised on the eighth day after being born. He could not do that if He was made at 30 years of age. Also, if He were not born, then Jesus would not be both divine and human because there would not be a true incarnation of the Word, which was God (John 1:1) and was made flesh (John 1:14) in the one person of Jesus. Also, if He were "made 30 years old," then He would not really be 30 years old. He would only appear to be 30 years of age. This is important because in order for a priest to be eligible for his priestly work, he had to be 30 years of age (Num. 4:3). So Jesus needed to be born under the law, be circumcised, be in subjection to His parents, and grow to the age of 30 before He was eligible to become the high priest after the order of Melchizedek and perform the atoning work.