Try moving your eyes in one direction as far as you can. You can’t go much farther than seeing the direct edge of your eyesocket. How come your eyes can’t move more?
You also said the answer yourself. “You can only go to see the edge of your eye socket”. It would have no evolutionary benefits for us to be able to move then more, because we are some of the lucky ones that have eyelids.
But short answer. The muscles controlling the eye, has a limit of how much they can move the eyes.
Your eyes moves thanks to muscle attached to the eyeball, so you can’t go further than the range of movement of those muscles. Also the optic nerve is located behind the eye, so it’s another limiting factor to eye movement.