Because I have kids and a wife that I love and I WILL NOT hear my children crying for food if I can do anything about it. I WILL NOT allow my children to become victims of criminals if I can do anything about it. Not everyone lives where such things as confiscation would be an easy task.
If this latest (2016) assault weapon (semi auto) ban passes... I will simply install hydraulically actuated rotating bolts, and jimmy slap triggers. They can't regulate what they don't understand.