Because the definition of criminal is someone who keeps laws.
It's about making it difficult to obtain them with licenses and bureaucratic hoops, and mandatory personality checks for gun owners perhaps on a yearly basis.
Outright banning them, however, or making it too difficult, just gives more power to the black market. It's definitely a fine line to walk, but it's one that needs walking and frequent tweaking to keep up with the times and with local culture. It can't be made easy for the sake of hobbyists' fun; these are real dangerous tools. Human life is more important than someone's fun.
One possible approach: require liability insurance for gun owners the way we do for cars (along with a hefty penalty for owning or carryinh a gun without insurance). If a gun owner really is safe and responsible, they'll find this reflected in low premiums. Riskier gin owners will find themselves priced out of firearm ownership.
And for those who think that guns make them safer, we should allow insurance companies to take the presence of a firearm (and a trained person to use it, obviously) into account when setting premiums for home and business insurance. if the gun advocates are right and they do make things safer, the actuaries will see this in their data and discounts could be given accordingly.