Armies don't typically fight unarmed opponents in close quarters. If you're unarmed and the attacker is within a few yards of you, a knife or a gun are not that different.

http://www.youtube.com/watch?v=9igSoJHEdUo

So lets ban guns then, people only need knives for self defence.

Self defense is not the only reason for the legality of guns.

Overthrowing government is the other I guess?

Good news is that their soldiers will have guns, and you can get a good advantage on them by staying in close quarters environments with your knifes.

Recreation. Target shooting. In countries where guns are not demonized, this is a perfectly acceptable activity.

I'm pretty sure they were waiting until the Supreme Court composition favored them before bringing a case. Heller was a 5-4 decision, filing when the court was more left/liberal could have had resulted in a devastating judicial precedent for gun owners.