As light travels through the air, it is subject to running into anything that’s in its path. Atoms and molecules in the air, including anything carried in the air like dust or smoke, will scatter light. Water droplets, as they are presented in fog, scatter light big time.
The light falling on an object and reflected to a viewer can be scattered to heck and back before it gets to the place where it can be “seen” by an observer. In the mean time, light from where the observer is can be scattered (and some can be reflected) so the observer just sees a “whiteout” instead of being able to make out anything beyond a few meters or so.