Why can't we make fresh water?

Look, we know water is composed of two parts hydrogen and one part oxygen. I can buy hydrogen in a tank and oxygen in a cannister do why don't we mix them together and make all the fresh water we need? Only 3 percent of all the water on earth is fresh water and of that only 1 percent is suitable for human consumption. We have vast areas on earth that lack fresh water. Even in America, one of the wealthiest countries on Earth, we have to make dams and reservoirs to contain fresh water for consumption.Why don't we simply set aside a billion each year and make our own fresh water? srw