For example, down vision angle at DH with a constant minimum visual range only small ALS and TLS segments are visible. When PAPI/VASI could be supposedly perceptible the airplane is just few feet above the runway or has already started a rejected landing due to several reasons but never because of PAPI/VASI indication. In short, under reduced visibility everything seems to happen very fast, so there's no time and PF is unable to evaluate airplane profile descent using external approach path cues.