If it's found in nature, it's not patentable, so no drug company will market it. They need to find a variation they can patent, so no one else can sell it.
Actually, the synthetic process is what is patented. Many blockbuster drugs are natural products, but they exist in nature in trace ammounts, not sufficient to supply the world demand. So they need to be created in the lab synthetically, as opposed to extracting them from biological organisms (plants, marine). The cancer drug Taxol, for instance, is a multi-billion dollar natural product that generated about $350 million in patent royalties.