Why cant underage buy cigarettes?

In the US 16-year-olds are not considered adults in the legal system except in very specific circumstances. Although in a few states a person of such age may file a petition to be emancipated, emancipation decrees are rarely granted. A 16-year-old can, in a few states be adjudicated by the court as an adult for committing a serious crime such as murder, arson with intent to kill or injure, kidnapping, and so forth,