because you are supposed to get the rest from the foods you eat. It's actually better to get as many vitamins and minerals from your food as possible. Vitamins are only supposed to supplement our diet- not replace healthy foods. Some vitamins don't even have natural ingredients. To be sure you are getting vitamins & minerals from food I get organic ones that say from a whole food source etc.
The best source is the sun. Makes you feel great. I suggest eating foods with things you want, rather than taking pills.
Because it's a minimum average. Everybody's needs are different, plus everyone's size is different. How can you accurately calculate a single pill that will fit the needs of both someone who weighs 90 pounds and someone who weighs 300 pounds?