Because some stars emit more blue and some more red. Also you can't have 100% light in universe (without creating a black hole that is), so when you mix lights,

Because some stars emit more blue and some more red. Also you can't have 100% light in universe (without creating a black hole that is), so when you mix lights, you have to mix different colors as unbounded values and then divide by max to get some normalized representable rgb value with 100% luminosity. #FFxxxx means there is more red than other colors, but only a little more. If every frequency in visible light would have the same strength in resulting spectrum, you would have #FFFFFF.