Because I've read history, including the Federalist Papers. Yes, the 2A enshrines the right of people to keep and bear arms, but the extremes to which the gun lobby has gone to interpret what that right means is absurd and foreign to anything the Founding Fathers would have ever imagined.
I have no problems with the 2A or the 1A - it's ideology that has corrupted the Constitution. Ideology is stupid, including some left wing ideology. The people who formed this country never wanted ideology, because they knew how fucked it was. They wanted a new kind of society built on the foundation of economic growth, liberty, civic duty, personal responsibility, and moderation. These are actually conservative values, but things people who pose as conservatives piss on at every turn.