This is because your scene has no indirect lighting, and presumably no environment lighting from the sky to light up shaded areas.
You need to set up some kind of global illumination solution such as GIProbe (GLES3 only) or baked lightmaps (GLES3 and GLES2). Alternatively, you can:
- Set the ambient lighting color to a non-black color in WorldEnvironment (and set Sky Contribution to 0 so it can be seen).
- Set the light's Shadow Color property to a non-black color to make shadows not fully black.
- Add shadow-less OmniLights or SpotLights in the scene at specific locations to fake indirect lighting.