There is a difference between "drinking a glass of water wouldn't hurt you" and "you are dehydrated and it is medically relevant". Every organism on this planet can easily cope with a reduction in available water if it isn't severe. The operating conditions of a human body, if formalized, would only state a minimum of water intake, because the optimal amount of hydration varies wildly from person to person, and it is also dependent on outside circumstances, like heavy work or high temperatures.

During the winter months, i am completely fine with around 1 liter of fluid intake per day most of the time when sedentary. During the last 2 months, i have topped out at around 7 liters/day (i do sweat a lot), which made me a bit nervous and i made sure to increase my normally very low salt intake a bit, to prevent running out of electrolytes.