Why is the flu shot advertised for all age-groups so aggressively when the chance of dying from the flu itself (not pneumonia) is astronomically low?

Several reasons: The people most at risk to die from influenza and its complications are usually the very young and the elderly. Both of these groups may not have a full immune response from the vaccine due to their age, so "herd immunity" is important in protecting them. For additional information here is a helpful link. http://www.who.int/mediacentre/factsheets/fs211/en/