TLDR, their optic nerves are coiled at rest, and uncoil when the eye is moving to extremes, which is different than the methods that other animals evolved to deal with the same issue. Owls pivot their heads. Rats have stretchy nerves. Chameleons coil and uncoil.

And in humans, I guess our anatomy is sufficient that it doesn’t need to move much to support our range of eye motion, so it just has a small amount of slack.