Because they have yet to head a sound argument to that end, with a specific and detailed explanation as to how said ban does not violate the Constitution.
Givie it a try. I dare you.
it's not overly complicated. one could simply argue that a magazine itself was not a weapon.
further, as a nation we've already decided that the second amendment isn't absolute. we don't allow people to own surface to air missiles, large bombs, or chemical weapons. we limit the arms people have a right to keep all the time.
the question then becomes where is the line and do or should large magazines cross it?
it is my opinion that since they serve no other practical purpose than to enable mass shootings they should be more closely regulated if not outright banned.
The 1939 Supreme Court ruling would disagree with you. In order to be protected under the second Amendment a weapon must be in use, of use or previously in use by the military. That clearly includes high capacity magazines.
A magazine is not a weapon. That aside your criteria only fixes a minimum standard. We clearly do not allow individuals to own all weaponry used by the military. Therefore simply arguing that large magazines are used by the military is insufficient to label them as absolutely protected.