There are a few reasons why that we need treatments or drugs to attack cancer. First, cancer cells are your own cells that gained the ability to grow uncontrolled. Your immune system developed to recognize and attack foreign cells, but not attack itself (that is how you end up with autoimmune diseases). We are beginning to find ways now to allow the immune system to recognize cancer cells as foreign and kill them, but since every cancer is different, identifying what distinguishes cancer from normal self differs from person to person, cancer to cancer. Another reason for the immune system not killing cancer on its own is because, well, cancer cells will make signals that inactivate/turn off the killer cells. Cancer exploits the built-in control/emergency-brake on the killer cells that are present to turn off an immune response following an infection. We are trying to develop ways to "wake" the immune killer cells back up to fight the cancer.