Because it's possible, to some extent, at no great cost. You don't render the universe.
Do you really think your computer handle the entire galaxy at once ? Or even entire planet ? It doesn't. No computer could (except perhaps some supercomputer). If you are in orbit of a world, the computer handle a predetermined random mesh and texture (and by mesh, it's basically a sphere). The skybox is pretty much the same. As you go down, that texture is replaced. Several time I think, until you get to the ground. Then the game only render the area in front of you, up to some distance.
See, we are very far away from "the entire galaxy".