Why is Sfumato more successful than solid outlines in creating realistic looking figures?

Sfumato, (from Italian sfumare, "to tone down" or "to evaporate like smoke"), in painting or drawing, the fine shading that produces soft, imperceptible transitions between colours and tones. It is used most often in connection with the work of Leonardo da Vinci and his followers, who made subtle gradations, without lines or borders, from light to dark areas; the technique was used for a highly illusionistic rendering of facial features and for atmospheric effects. See also chiaroscuro. (got from google

Sfumato, (from Italian sfumare, "to tone down" or "to evaporate like smoke"), in painting or drawing, the fine shading that produces soft, imperceptible transitions between colours and tones. It is used most often in connection with the work of Leonardo da Vinci and his followers, who made subtle gradations, without lines or borders, from light to dark areas; the technique was used for a highly illusionistic rendering of facial features and for atmospheric effects. See also chiaroscuro.