Because you continue to stick your head in the sand about self defense.Because the real objective is to reduce the number of people who get shot.You'll have to take out an awful lot of legal guns to reduce supply of illegal ones. Why should we not believe that the primary effect is the real objective?
If the number of legal guns falls dramatically, what problem does this cause for anybody?
We do not have good data on self defense incidents, but it certainly looks like a legally owned gun is quite a bit more likely to prevent a crime than commit a crime. (And note that most defensive uses of guns don't even involve drawing them. Somebody realizes they're being set up as a target, they reveal their gun, the bad guys take off.)