Why did God send Jesus a baby? Why did he not send him as an adult? He could have done that as he did with Adam and Eve. It would have been much safer.
The problem if Jesus had come as an adult is that the spiritual powers of evil could argue that Jesus was not truly human and not entitled to authority on earth. He had to be born helpless and grow to be a man, so there could be no argument about his humanness, and his right to take authority over the earth. He had to be a true descendant of Adam. Being born to an Israelite girl was the only way that it could happen.