Easy. Many people don't know how important it is to drink. Even if you do know you may be exhausted to the point where you cease to do the right thing - i.e. to tired to think, too tired to drink.


On a regular day hike it takes a conscious mental effort to drink enough - at least for me. I can very well imagine that the effort required to drink enough is trippled when that day hike turns into a survival episode. Never been there, but there are plenty of stories that confirm that this is actually so.