Because those don't actually do anything. A high capacity magazine is less sophisticated than a beer can and we already have two centuries of supply in circulation, and reloading takes about two seconds. Assault weapons are used in mass killings only a sixth as often as a cigarette lighter, and are not as effective at killing people. Post Heller, a gun ban is clearly unconstitutional, and given the UK's example, would just mean that only violent criminals and the mentally ill would have guns.
The mentally ill would not have guns, take this Adam guy in the UK the only way he would have got hold of a machine gun is if he knew the right people and considering he was a complete loner than that would gave been difficult.