Sfumato is more successful than solid outlines in creating realistic figures because it relies on subtle gradations of tone and color, mimicking the way light interacts with surfaces in real life. This technique allows for smoother transitions between light and shadow, enhancing depth and volume. By avoiding harsh lines, sfumato captures the complexity of facial features and textures, resulting in a more lifelike representation. This soft blending creates a sense of atmosphere and intimacy that solid outlines often lack.