“There is no way that taking a pill can replace eating fruits and vegetables,’ writes Prof Walter Willett Chairman, Department of Nutrition, Harvard School of Public Health in Eat Drink and Be Healthy. ‘So far, no one has found a magic bullet that works against heart disease, cancer and a host of other chronic diseases as well as fruits and vegetables seem to do. In theory, one could cram all the good things that plants make—essential elements, fiber, vitamins, antioxidants, plant hormones, and so on—into a pill. But it would have to be a very large pill, and no one can honestly say what should go into such a pill or in what proportions.”
Also, human body is a complex machine. We need the combination of small amount of multiple nutrition rather than large amount of one single nutrition. The benefits of eating fruits and vegetables probably come from combinations of compounds that work together.
So, when you need Vitamin C, why not just eat an orange?