What is the primary reason the faint, scattered starlight reaching Earth at night often fails to register as distinct illumination?

Answer

The faint, scattered light is often just below the perception threshold of human vision.

The human eye has an absolute threshold below which it perceives nothing; the faint, scattered light from billions of distant stars that crosses space often falls below this level when the bright light of the Sun is absent.

What is the primary reason the faint, scattered starlight reaching Earth at night often fails to register as distinct illumination?

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