Every Tom, Dick, and wife-beating Harry doesn’t need to walk around with enough firepower to massacre a neighborhood.
The Constitution is a framework of government, not a goddamn suicide pact. Society and technology have changed since it was written, and we aren’t worried about needing the family musket to form a citizen militia to repel the Brits invading from Canada. And even by the end of the Revolutionary War, the myth of farmer militias gave way to the reality of a professional army.