The question is clear and I am not sure how to further explain it. Why don't cameras have internal automatic eyepiece covers?
By "internal automatic eyepiece cover" I assume you mean something to block the light instead of something to protect the eyepiece glass from scratches.
There are no cameras with an "Automatic Eyepiece Cover" but some Pro level cameras have a manual eyepiece shutter to block off light during long exposures when your eye is not covering the viewfinder. This prevents exposure errors due to light entering the viewfinder.
Other Cameras use a rubber piece that snaps onto the viewfinder to block the light.