Because there are ....guns.
Simple.
No guns, no gun violence.
Of course, with 300 million guns in this country, it will be hard to collect them all.
If the 2nd Amendment was adhered to as WRITTEN, no individuals would own guns, other than hunting rifles. And, we'd have much less "gun violence". Just like everywhere else in the civilized world.
The problem is all the existing guns. And it won't go away, no matter what.
Load up, dudes.