Many players play table tennis like they were playing pinball. They stand there, and when they have to move, they mostly just reach for the ball. The reason for this is a combination of two factors: they are not ready to move, and they are not in the habit of moving. These are two separate things.
A coach could stress the proper ready stance and the importance of moving, and try to get the player to be ready to move and to develop the habit of doing so. And it might sink in. But there's an easier way. Put your racket aside. Now imagine you are covering someone in basketball. (Or an infielder in baseball or a goalie in soccer.) Don't think about it; just do it. Then examine what happens. Almost for certain your feet went a little wider, your weight went on the balls of the feet, your knees bent some and pointed inwards slightly, and you leaned forward slightly at the waist. Your knees were suddenly bouncy as you prepared to move in either direction. (You probably raised your arms as well; you can drop them now, but keep the free hand up some for balance.) You are now in a perfect ready position!
You are now ready to move. Do this regularly, and you'll develop the habit of moving. Your pinball days are over.