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.

Related Questions
Why does the sky appear blue during the day on Earth?What is the primary reason space appears inky black when viewed from orbit?What process is responsible for bathing the Earth's environment in diffuse blue light?What concept describes the difficulty cameras have capturing both the bright white of a spacesuit and the faint background stars?Why do photos taken from the sunlit surface of the Moon show a pitch-black sky?If you held up a pane of glass in space, what would happen to starlight passing through it?What is the fundamental reason stars are often invisible during a daytime EVA?Under what condition can astronauts immediately see stars while performing an EVA near the brightly lit Earth?How does the blackness of space affect the observed intensity of colors on illuminated objects like Earth?What causes sunlight to be redirected toward an observer's eye on Earth, but not in space?