Very simply because our bodies are about two thirds water. The human body contains about eleven gallons of water, and as we get rid of our body water by sweating and other means, we must replace it. The water in our bodies is not the same as ordinary drinking water, of course. There are many chemicals in it, which the body needs to grow and repair itself, and the water moves these substances around inside the body so that each part gets the amount it needs. Although on some days you may not drink as much as on others, you take in water from the solid things you eat vegetables, fruit meat and bread are about a third water themselves.
Blood is mainly water. About a gallon of all water in our bodies is in the blood, but that level always remains the same, no matter how much you drink.