Spheres don't really exist in computer graphics. Look close enough at any "sphere" created by a computer and you will see polygons, this is inevitable due to the limited resolution and computing power that is currently possible, and antialiasing will only blur the sharp edges of the polygons, it won't fix it, you will always be able to tell a sphere is composed of polygons. Increasong the number of polygons makes the surface smoother but also is harder to process, meaning there will be more lag. Comprimises must be made, and the developers have decided to give us good performance rather than an insanely high polygon count which many users would not even notice.