This question belongs to all food products and medicines that why it is needed when nature has provided in more balanced and natural form.
It seems like a philosophical question with an answer inside to another question, whether that's true all of the time is another more interesting question.
Basically the answer is evolution.
The more interesting question is, "when isolated substances are better than their natural existences and why..."
Paleo: appropriate after gastric bypass? 12 Answers