They didn't write it that way because that was not what they meant. That would be like writing a lot for nothing, that was appearant and obvious. It is also obvious to any person that reviews the well documented discussions they had at the time.
What they wrote was that the ability of the people to have all the tools, including arms, necessary to assemble into organzied groups of rebels shall not be infringed.
That was much more controversal than the right to just have the arms, as was already covered under English Common Law.
The right to bear arms was guaranteed in the 1689 Bill of Rights, in which the new King William of Orange enshrined a series of rights for his subjects.
One only need look back to the events of the Magna Carta or even thier recent rebellion from England to understand why the founding fathers felt it needed to be taken a step further.
So if written today it would instead mention becoming "insurgents".
Yes it is very radical, and is what they meant. They wanted to insure the people's right to arms was never infringed, so that if necessary they could band into random local bands, or militias/insurgents and stand up to tyranny when necessary.
That is why the term militia was important in the 2nd. It wasn't about keeping a firearm capable of standing up to the local thug, it was about keeping arms capable of standing up to even the military troops of tyrants.
So I repeat, English common law already provided for the right to arms, the 2nd took it a radical step further and said the right to arms could not even be reasonably restricted or "infringed" upon in order to insure the preservation of the ability of the citizens to become insurgents when necessary. Something no kingdom in the world previously had said. Can you imagine a king telling his subjects they could possess arms in order to kill him or his men if they needed to?! It was an unprecedented right, and so important that it was second only to the first amendment, protecting people's right to discuss and believe whatever they wanted to.
At the same time they believed the government's job was to fight those insurgents when they rebelled, as was seen in various rebellions Washington put down. So they were not saying people would not face consequences for becoming insurgents, just that they should never have restrictions placed on the tools that enabled them to do so otherwise the balance would be compromised, and the people would be vulnerable to tyranny.
Yes it is complex, almost contradicting in today's black or white desires. However they did not picture governments as perfect, and acknowledged that even the best could at any time become corrupt and self serving. So the population needed to be perpetually armed and capable of taking on the government when necessary.
So it really is not about the right of individuals to have arms alone, it is about the right of individuals to never have thier ability to keep and bear (not just own, but use) arms infringed on in any way for a very specific purpose: To be capable of fighting tyranny with force, both in the form of a foriegn invading force, or agents of the people's own government.
So stating the right of individuals to keep arms would not have been unique at the time, even Englishmen could legaly carry a pistol in thier pockets, and many gentlemen did just that.
In fact england did not have that right restricted until 1870, when they started requiring people to obtain a CCW license to legaly carry a pistol outside the home.
33 years later in1903 England passed a law restricting "drunken or insane" people's right to be armed. Sound familiar? It should, the exact thing today is written as "...ever been commited to..." "...unlawful user of or addicted to..." on the 4473 form.
17 years later England had a law passed along the lines of "unfitted to be trusted with a firearm" allowing police to deny such people the right to arms.
So you see at the time England allowed anyone to be armed, and only later did the go down the same road we are going down now before arriving at thier gun free utopia. So why would the founding fathers, who still used English Common law which already protected the rights of individuals to be armed specificly state that same thing again?
They wouldn't and they didn't. They said it could not be infringed upon so the people could become insurgents. Something totaly new and different, and not previously stated in Common Law.