Because we save far more patients than we kill. Firearms are entirely analogous. Firearms, like physicians, prevent far more deaths than they cause. That means decreasing the number of firearms would actually cause an increase in violent crime and deaths from firearms. This inverse relationship between the number of firearms in the hands of the public and the amount of violent crime has, in fact, been proven beyond any reasonable doubt.
History supports the inverse firearm-crime relationship.
Half a century of strict controls on pistols has ended, perversely, with a far greater use of this class of weapons in crime than ever before. In Tasmania, Australia, on 28 April 1996, a lone gunman killed 35 and wounded 21 at the Port Arthur Historic Site. One year after the massive confiscation of guns the effects of this action became clear. Such gun-restrictive proposals are a certain recipe for making the situation worse. Consider the steadily decreasing rate of violent crime over the past eight years.