What is the fundamental reason stars are often invisible during a daytime EVA?

Answer

The direct and scattered light from nearby sources overwhelms the faint, point-source light of the stars

Stars do not disappear; their light is simply overwhelmed by the immense brightness contrast provided by the nearby Sun, or the reflection off the Earth or Moon, making them invisible until foreground light is blocked.

What is the fundamental reason stars are often invisible during a daytime EVA?
Spaceskyastronautsvacuumlight scattering