Because early in the history of life some cells evolved the ability to photosynthesize while others got their energy from organic molecules and later from the molecules that the photosynthesizers produced. Its a basic producer / consumer split in the history of life. We descended, like all animals and fungi from the line of non-photosynthesizers. Therefore we are not self feeding autotrophs.
It's a good thing too I suppose. Otherwise we would not have complex nervous systems with the ability for thought. An organism as complex as yourself would not, even if your epidermis was covered with chloroplasts, produce enough energy for you to do what you do every day. You'd just be able to lay in the sun all day (probably not even that... You just wouldn't/couldn't exist as an autotroph)
