Because the right to self defense is a human right, which is acknowledged in the Constitution as the "inalienable right to life, liberty, and the pursuit of happiness". And this shows that they knew this, and the government cannot "give" us rights that are naturally ours by birth.
That's a good answer, but it's only part of it.
The Founding Fathers did not believe we got our rights from the Bill of Rights. Nor did they believe they came about as a result of being American, Christian, of European decent, or white. They believed everyone had these rights even if they lived in Europe, China, or the moon. They called them Natural Rights. Where these rights were not allowed, they believed they still existed but were denied.
Our rights exist apart from government. Let me ask you this, in a country where children have no civil rights, do they still have a right not to be molested? Do women in countries where they have a second-citizen status have the right not to be abused by their husbands, even if the government wont protect them?
If the government passed a law tomorrow that said we didnt have the right to free speech, or the right to free worship, or freedom of the press, would those rights no longer exist, or would they be simply denied? If the Constitution is amended depriving us of our rights, do those rights cease to exist?
According to the guys who set up this country, yes, we would still have those rights. Were just being denied them. Because of that, its the way we have to look at the Constitution.
It may be, that in reality, rights are a figment of our imagination. But the Founding Fathers believed they existed and thats how this country was set up. Rights are something that come with being human. The Founders never believed we got them from the government. If and when the United States goes away, the rights will still be there.
Then why have a Bill of Rights?
Men like Alexander Hamilton asked that same question. He and many others thought having a Bill of rights was dangerous. They were afraid that the existence of a Bill of Rights as a part of our Constitution implied that the government not only had the right to change them, but that any rights not listed there were fair game for the government to deny. And, as a matter of fact, thats exactly what has happened. The government seems to have set itself up to be an interpreter of our rights; it acts as if it is also the source of our rights, and whatever rights werent mentioned in the Bill of Rights, the government has seen fit to declare exist only at its discretion.
The 9th and 10th amendments were added to the Bill of Rights to quell the fears of men like Hamilton who were afraid that any rights not mentioned in the Bill of Rights would be usurped by the government. The 9th says:
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.
This means that any rights not mentioned in the Bill of Rights are not to be denied to the people.
The 10th says:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
So any powers not specifically given to the Federal government are not powers it can usurp.
So its enough to show the Founding Fathers thought we had a right for it to fall under the protection of the 9th or 10th Amendment. This means that the Founders didnt even have to specify we have the right to free speech, religion, jury trials, or anything else. To understand what they felt our rights were, all you had to do was show what they said our rights are. Any rights in the first eight Amendments are just redundant with what the Founding Fathers considered Natural Rights.
Aside from the gun issue, your rights are being violated right now:the Founding Fathers felt we had a right to unrestricted travel. So, now we have drivers licenses, automobile registrations, and passports. They also felt we had property rights, so Civil Forfeiture or Civil Seizure laws, now exercised by the Feds and the states, are actually illegal under both the 9th and 10th Amendment.
I'll add here that no where does the 2nd Amendment deny the people "the right to keep and bear arms," so, even if the Congress or even the Supreme Court decides the 2nd Amendment only refers to formal military organizations, we still have the right to keep and bear arms, because the Founding Fathers considered it a natural right. And if you dont believe it, read what the Founding Fathers said in their papers, their letters, and their debates in both Congress and the state legislatures
(Weapons have always been important. In Greece, Rome, and even under Anglo-Saxon law, when slaves were freed, part of the ceremony included placing a weapon in the mans hand. It was symbolic of the mans new rank as a free man.)
John Adams, A Defense of the ConstitutionArms in the hands of individual citizens may be used at individual discretion...in private self-defense.
Samuel Adams (yeah, the beer guy), John Adams second or third cousin, during Massachusetts U.S. Constitution ratification convention in 1788.The Constitution shall never be construed to prevent the people of the United States who are peaceable citizens from keeping their own arms.
Thomas Jefferson, with the last as part of the proposed Virginia Constitution of 1776.The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in Government.
No free man shall ever be debarred the use of arms.
Jefferson, again quoting Cesare Breccaria, a Milanese criminologist in On Crimes and Pnishment.Laws that forbid the carrying of arms...disarm only those who are neither inclined nor determined to commit crimes...Such laws make things worse for the assaulted and better for the assailants; they serve rather to encourage than to prevent homicides, for an unarmed man may be attacked with greater confidence than an armed man.
Thomas Paine, in Thoughts on Defensive War, 1775Arms discourage and keep the invader and plunderer in awe, and preserve order in the world as well as property. Horrid mischief would ensue were the law-abiding deprived of the use of them.
George Washington, speech to Congress, January 7, 1790Firearms stand next in importance to the Constitution itself. They are the American peoples liberty teeth and keystone under independence. From the hour the Pilgrims landed, to the present day, events, occurrences, and tendencies prove that to ensure peace, security and happiness, the rifle and pistol are equally indispensable. The very atmosphere of firearms everywhere restrains evil interferencethey deserve a place of honor with all that's good.
A personal favorite:
To disarm the people; that it was the best and most effectual way to enslave them.
This last was from George Mason, during the debates on the Constitution itself