Because tap water that has not been purified is contaminated with all sorts of bacteria and harmful stuff would drinking 4 to 6 liters of tap water that is not purified be extremely dangerous enough to make you dangerously sick to the point where it would result in having to take a ride in a ambulance and be presented to the emergency room because I have read online about the dangers of drinking tap water that is not purified and that why it’s very important to purify it by using a reverse osmosis or by boiling it on a stove top