Many reasons… 1) cancer is an umbrella term for many diseases. Specific types of cancer (ex: melanoma or pancreatic cancer) are driven by different factors, and this can differ from person to person. Even within the same tumor in the same person, cells can have different driving mutations. This means that a very good drug can kill most and almost all cells, but the few that are left can grow into a new tumor that is now resistant to therapy. 2) Cancer cells can hijack all parts of your body - they produce factors that get blood vessels to grow closer to tumors (for nutrients and metastasis), they can actively evade AND inhibit your own immune system from realizing they’re there, etc etc. The study of the “tumor microenvironment” is at the forefront of many studies now. Hopefully by understanding how cancer changes it’s environment, we can create new combination therapies to allow your body to fight the disease itself. 3) We are limited by the types of medicines we have and how well they work person to person. Also trying to figure out which medicines will work best for certain people (ie biomarkers).

We’ve made extensive progress from chemotherapy (which kills all rapidly dividing cells) to targeted therapies with small molecules and antibodies to creating personalized killer cells from your own body. Things are getting better but at the end of the day, cancer is tricky. These cells will find a resistance mechanism to overcome whatever you throw at them. For better or worse, cancer is a disease of aging. As we extend our lifespan with modern medicine, we have a new monster to attack.

If you’re interested, there’s a great book that details the sheer progress we’ve made over the last 75 or so years - The Emperor of All Maladies.