Because the mindset behind the modern, health conscious version of abstinence does not stop at alcohol. It tells you to monitor and optimize every single parameter of your health, avoid any substance that may or may not damage your body, just so you can maybe live five more years - five more years spent doing what exactly?

It's a paranoid way of living and frankly people are likely doing more harm to their health by being so anxious about ingesting one nanogram of carcinogens than the carcinogens themselves.