The only man who ever walked the earth and was not born of a women was the first man, Adam. All others have descended from him through childbirth. This includes the Christ. Sure, God could have simply materialized in the form of a man like the angels do, but He would not have been a member of the human race. To be a genuine human being, He had to be an actual descendent of Adam.
Perhaps the passage that best descibes why God chose to come this way is Hebrews 2:14-18, which says, 'Inasmuch then as the children have partaken of flesh and blood, He Himself likewise shared in the same, that through death He might destroy him who had the power of death, that is, the devil,15 and release those who through fear of death were all their lifetime subject to bondage.16 For indeed He does not give aid to angels, but He does give aid to the seed of Abraham.17 Therefore, in all things He had to be made like His brethren, that He might be a merciful and faithful High Priest in things pertaining to God, to make propitiation for the sins of the people.18 For in that He Himself has suffered, being tempted, He is able to aid those who are tempted.'
If God had not been a real man, we would not be able to accept that He really understands what it was like to face temptation. Frankly, I can't relate to Superman. Furthermore, His purpose in coming was not to give aid to angelic beings who seem to go on forever, but to those of us who are human beings, subject to temptation and in bondage to death.