When you form your embouchure,your lips are touching inside the mouthpiece. As the air passes through your lips,the aperture is then formed. You can't control how open or closed your aperture is for each and every note.
This would mean you could only play as low as you can open the aperture ,and only as high as you can close it. So once you close your aperture all the way you will hit a wall and will never get passed it
If it were only a matter opening and closing the aperture most of us would never play past 3rd space C. After all once the aperture is closed it's closed.This would also make playing low notes at ppp impossible.