Because otherwise they would be unable to track movement and move the cursor in the computer screen.
A hole-y Cow!
In case he got a hole in one.
An Appaloosa!
I replied, "I'm not sure, it's hard to keep track"
Eyes down for a full mouse' !
I said to watch him like a hawk! ME: soaring 20m above w/ a beakful of mice I AM
His vision was based on movements.
Because your foot would go right through the computer screen!