Is there a purpose for the hourglass shape being beautiful? Symmetry and contrast could be reasons. But to the question of why specific people have to be beautiful and why some are not, well, it's just God's glory being manifest in this world in that way just because God wills it so. And They want us to love the arbitrariness of the assignment to an extent, but that we should also hope to see in these beautiful people models of what is possible for many others, if given resources and time. It is never a dead-end "she's beautiful" only. It is meant to give us a preview of the instantaneous recognition of God's beauties on Earth and how much humans have it good when it comes to God, who designed beauty and order and rationality to be pleasing to not only the eyes, but to the heart and mind too.