"Why haven't they invented a "healthy" cigarette, or at least one that isn't quite so full of carcinogens?"

Well, cigarettes are a drug which is deliver via smoke.

I doubt there is any drugs which don't have carcinogens.
For instance:
"According to the expert report from the American Institute for Cancer Research, Food, Nutrition and the Prevention of Cancer: a Global Perspective, there is convincing evidence that alcohol increase the risk of cancer of the mouth, pharynx, larynx and esophagus. The risk of upper respiratory tract cancer is greatly increased if drinkers also smoke. Alcohol also increase the risk of liver cancer and probably increases the risk for colon, rectal and breast cancer."
http://www.healthcastle.com/alcoholandcancer.shtml

Of course alcohol actually results in more deaths than cigarettes. Alcohol deaths also generally more direct and without any doubt about the cause.

Next you have the problem of burning anything and inhaling the smoke and not having carcinogens- good luck with that.
If you think that smoking dope, doesn't have any carcinogens in it, that's because you are smoking dope.

Also, if cigarettes weren't addictive, people would probably forget to smoke cigarettes. And probably if gambling or eating sugar was not in anyway addictive these things would done less.