Its a Bill or Rights, not a Bill of Needs.
So the question is not why we should allow it.
The question is (a) upon what factual or legal basis should we ban it and (b) is the political cost of calling for bans and confiscation worth it?
What keeps me awake at night is not the likelihood of getting shot by a lunatic with a rifle. Gun homicides have decreased 45% since 1991, and rifles of all kinds are used in less than 5% of all shootings. What keeps me awake at night is my wife losing her health insurance over pre existing conditions, or my gay sister, transgender niece or non white relatives being targeted for hate crimes due to foolish calls for gun bans that result in 4 additional years of Trump.