Except it isn't.
Most people, even many pro-gun people, don't realize the Constitution does say precisely that. Keep in mind that George Washington used artillery that was loaned to the Revolutionary Army by private citizens in the colonies. Many towns had artillery clubs, wherein the members maintained pieces of artillery. Plantations bought cannons in order to protect their property from Indian attack. Ship owners bought muskets and cannon to protect their ships from pirates. Many of these private citizens loaned their pieces to Washington under the proviso that they would be returned when the war was over - which they were, insofar as was possible.
George Washington was our first President. When we think "George Washington," we think "government", but that's not correct. The American Revolution and its Washington-led army was really just a bunch of armed citizens. These citizens were running around with the deadliest weapons produced at the time, weapons they purchased themselves, and all of these private citizens were shooting at government employees. That's the American Revolution.
That's the mind-set one has to have to read the Constitution correctly. We have to remember: from the Founding Fathers' perspective, armed citizens shooting at government employees was fine.
And, we should also notice that private ownership of weapons is not just a 2nd Amendment right - it is also in the body of the Constitution.
Article 1, Section 8: [The Congress shall have the power] To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water.A "Letter of Marque" was a contract between a private citizen and the government that authorized private ship owners to use their ships to capture shipping from the enemy. A Letter of Marque allowed a private ship to act as a warship. Without the Letter of Marque, the ship owner would just be a pirate. With the Letter of Marque, he (and his ship) became a mercenary for whatever government he contracted with.
But the Letter of Marque did not equip the ship - that was done at the owner's expense. Most ships at the time carried some kind of defensive cannon and musketry for protection against pirates and mercenaries. The ships were already armed. The Letter of Marque simply protected the ship in case of capture, assuring the captain and crew that they couldn't be hung as pirates. As for terms, the letter would primarily describe how the profit from successfully capturing an enemy ship was to be split between the government and the contracting ship owner. The Letter of Marque was really just a government rental agreement for the use of a privately-owned warship.
Why would the US Constitution do this? Because the Founding Fathers wanted the central government so weak that the governement couldn't even keep a standing army or navy. The Continental Army and Navy were both disbanded as soon as the Revolutionary War ended. Thus, the Constitution was ratified in 1789, but the Department of the Navy was not created until 1798, in order to combat Muslim Barbary pirates. For the first decade, the Congress expected they wouldn't need a standing Navy. Letters of Marque, government rentals of private warships (at least 2200 private warships were issued such letters), were supposed to take care of that need. Indeed, as late as July, 1815, Thomas Jefferson issued a Letter of Marque to the Grand Turk, authorizing the Grand Turk to operate against the Barbary Pirates on America's behalf. Although the United States government gave up issuing Letters by 1856, it continued to honor the Letters of Marque issued by the Confederacy during the Civil War.
But, in order for the Letter of Marque to even be listed as a possibility in the Constitution, the Constitution has to assume that private citizens had already bought and were already using the most powerful weaponry then available: fully-equipped warships. Put bluntly, Article I of the Constitution shows that the Founders originally intended to rent military power from private citizens. A similar agreement today might have a civilian purchase a tank, F-16 or nuke, and then rent the use of these weapons out to the US government. The Second Amendment has to be read within the context of the full Constitution and the full intent of the Founding Fathers. Once it is, there is no question about every citizen's rights.