Because he is running at 8 MPH in relation to the treadmill surface, not in relation to the room or the air around him.
EXACTLY MY POINT. Perception is the key to the question.
According to Jim, he will say “I’m running 8 MPH” because his view point is between his body and the treadmill surface.
Where as your view is that of Jim and the air in front of him and the room around him.
You can still say “Jim is running 8 MPH on the treadmill”
You cannot say “Jim is running 8 MPH across the room”
Get it?