Why do you have to drink sooo much water in order to lose weight?

Critical to function. Water is critical to our functioning. Water causes the stomach to feel full & thus helps to suppress appetite. It is required for lipolysis or the burning of fat. Water intake can help ^ metabolic rate. Water is required to cleanse the body of waste materials.
Hydration is VIP. It takes more water to burn fat than it does to burn protein or carb. If your body is even slightly dehydrated you will derail fat burning and burn the other "fuels" instead. Most of us are slightly dehydrated anyway so it seem to take a lot of water - some to get you adequately hydrated and then more to burn fat.