Comment by daenz

Look at the timeline:

  1930  - first cigarette company uses physicians in their ads
  1950s - evidence starts mounting that smoking causes lung cancer
  1964  - US Surgeon General report on the link between smoking and cancer
  1998  - cigarette companies still maintained that the link is controversial

So it takes 70 years, or nearly an entire generation, before all of the machinery at play (businesses, government, healthcare, scientists) can effectively come to the conclusion that they messed up badly and sold people poison. Grim.