The reason is because the aperture is now controlled by the camera.
In order for the camera to access the full range of apertures from F1.4 to 16, you need to set the lens to F16 so that the camera can then do the adjustment. Lens construction is such that if nothing is done, it will spring to the smallest aperture, F16. By pushing the aperture leaver, progressively larger apertures can be obtained.
In the manual days, this lever is operated by the aperture ring. In the digital days, this lever is operated by the camera. If you set at an aperture larger than F16, say F11, the camera can never set apertures smaller than F11 because the lever already sets the minimum bar.