Because we have a Bill of Rights, and not a Bill of Needs. We are a Country of free men and women, and freedom is a misnomer when it comes to the designs of others. It must be jealously guarded and protected from those who would corrupt those freedoms or take them away completely.
Our founding fathers were students of history and political science, and fully understood the ramifications of the saying, "power corrupts; absolute power corrupts absolutely".
Our Nation was formed as a Republic, not a Democracy. Our foudners also understood the evils of the tyranny of both the majority and a miniority in power. A government of limited powers exercisable only pursuant to and under a specific grant.
Our founders also understood and feared the presence of a large standing army, that could be used by a despot to opppress the people. To counter a standing army, or to take the place thereof, it was considered the duty (but not the obligation) of every capable man to arm himself and serve, when needed, as a member of the militia.
Some may think with our military forces, we have no need of a militia. That could be no further from the truth. Without the concept of the militia, there is nothing serving as a deterrent to themisuse of a standing army.
More improtantly today, our current President prior to the last election stated that it was his goal to have a non military force as well equipped and with as many resources as our military. And we are there - look at Homeland Security and the other federal agencies. Those employed by agencies such as the Department of Education, the Library of Congress, the IRS, NOAA and other seemingly innocous agencies are permitted by law to carry firearms. With no training.
Then we have the massive growth and consolidation of powers by agencies like Homeland Security, and their massive acquisition of military equipment and munitions. In other words, we now have something our founders never considered: A heavily armed federal force under the direction and control of the Executive Branch of government, without the military discipline or the well regulated training envisioned for the militia.
The performance of these agencies at Waco and Ruby Ridge, to name two incidents, are good illustrations of heavily armed forces unchecked by military command and control.
There has never been a greater need for the Second Amendment, nor those civilian men and women willing to protect our freedoms.
It has been said that we would probably never need a Second Amendment until someone tried to take it away. And as the Second Amendment goes, so goes our Constitution.