Guns and Knives are synonymous with everyday murders and gangs.
Most country's aren't constantly dealing with grenades and hostile rockets on a daily basis so they don't hit close to home like the former do.
Actually, if you wanna get into Geopolitics it's more fair to argue that a large portion of the world today is indeed deadlocked in war and endless strife, in which many people do indeed see 'rocket launchers and grenades on a daily basis.' However, those people don't buy video games, and the people that do pay no attention to the suffering happening in other countries, nor do they realize that it is that precise suffering that allows the to live in wealth and peace.
I would even go as far as saying that having rocket launchers and grenades is much more insensitive to the real problems in our world today than having handguns and knives. However, the world is littered with hypocrisy and double standards, and most of the world is numb to the reality of the situation because it is far too dark for most to handle, so instead they shut their eyes and look away. But turn on the news every once in a while, read about what's happening in the world these days, and what's predicted to happen in the coming years, it's not a pretty picture.
I didn't mean to get all preachy there, and I certainly got off-topic. But the fact remains that videogames are escapism. Censoring the content you can an cannot have in a virtual space because it is insensitive to the real world is the most bull**** thing I've heard get thrown around these days. It is about as ridiculous as telling people what and what not to fantasize about. Censorship is never the answer, quite the contrary it is the antithesis of progress, shutting your eyes and refusing to accept the reality of things is not the way to solve the problem. Of course, this is something that goes beyond what developers can do, and reaches more into the territory of not only modern politics, but our stupid and fickle modern society.