Because harsh light is not just a global contrast problem. It creates hard-edged, fast transitions between highlight and shadow, and those transitions depend on the 3D shape of the subject and the direction/quality of the light.

Software can brighten shadows, lower contrast, and do some local adjustments, but it usually cannot know which dark areas are true shadows versus real texture, color changes, or object boundaries. A normal photo does not contain enough scene-depth and lighting information to realistically rebuild how soft light would have wrapped around the subject.

Another limit is capture: if harsh light caused clipped highlights or blocked shadows, that missing detail may not be recoverable.

So editing can often improve harsh light, but it usually cannot truly transform hard light into natural-looking soft light without heavy manual retouching. Also, “harsh” light is not always bad—it can be a creative look depending on the subject and intent.