Even with an open stance you must get out of the way, get to the side of the path of the ball. The best analogy I can think of is a bullfighter. The bullfighter quickly sidesteps the bull as the quickest way to avoid it. You must learn to do the same with a tennis ball. Think of it this way.
Many players move plenty, they just move in the wrong direction.
Our first instinct is to park ourselves behind the path of the ball. A player hurries their swing hitting it out in front of their body as they frantically try to fight the ball off as it comes straight at them. Frequently this player will take two or three steps back in a vain effort to buy themselves more time. I have often asked these players why they are taking so many steps back. Hitting the ball out in front is a crutch for players that position themselves behind the path of the ball.