standard answer is they are not bright enough compared to the earth/other objects in view, so "iris" closes to white balance, making stars too faint to register.

Same reason you cannot see into your neighbours windows in the daytime, but at night when they have their light on, its no problem, even though in the day, there is more absolute illumination within their home due to daylighting, it is too little in comparison to outside and your eyes will be like pinholes unable to render the interior of their home.