Verified answer

Because it's a lucrative industry. And.. certain powerful people can legitimize the "demand" for such weapons to increase in a way that you cannot in the agricultural, electronic, etc sectors of the marketplace.

This may be due to existance of some trait of aggressiveness that resides still in us!

You can ask your self why men watch wresling, indulge in boxing, bull fighting etc

These all are risky and gives nothing but pain! But why do we do it?

Well it is for that thrill, that adrenalin rush, fullfilling once most basic of instincts. Thus I believe that though we say we are not cavemen, there exist yet some trait of him dominant in each and every one of us!