[dayman]

You're right that - in a perfect world - there would be no need for guns. But - sadly - we do not live in a perfect world, and many of us here would rather be prepared when things get ugly.
You're also right that we can't defend ourselves from everything. But, the same argument could be made against seat-belts, fire-alarms, or getting your yearly physical exam.

I think it's primarily a cultural issue.
Constitution aside, Americans come from a culture founded by revolutionaries. Since the founding of our country we've been citizens rather than subjects. Because of that, many of us reject the idea that the only those who work for our government should be able to arm themselves.