Well, here are a couple of reasons:
- Why should I, a sane, balanced individual who has never harmed anyone in my life, not have such a weapon for my defence or indeed my pleasure, if that is how I want to spend my time? I mean, people collect stamps or do morris dancing or watch cookery programs for fun. I can't understand doing any of those things for fun, but it doesn't mean I think they should be banned.
- "Nobody needs more than 30 rounds to defend themselves." Perhaps if you're John Rambo, or Ethan Hunt; but in the real world, things are very different. It's surprisingly difficult to shoot a moving target, even for trained professionals like soldiers and the police. I was once involved in someone running a road block with a car, where around 80 rounds were fired and only 3 hit the car, none hit any of the occupants. (I wasn't shooting mind, if I had been, no rounds would have hit the car!)
- "There is no need for anyone but the government to have such powerful weapons." This fundamentally misunderstands the skeptical view that the Founding Fathers had of government. It is precisely because the government has such powerful weapons that the common man should have them too, to be able to bear equivalent arms against the state. You need to keep the state's monopoly on violence as weak and counterbalanced as possible.