I feel this is because the US says it is fine for people to own firearms. So there are many people who go to an extreme and instead of buying just one, they buy 20. Then we have gun collectors who love to collect guns, a gun collector can own up to 50 or more guns in his collection.
Now if we are talking about the military, they feel the need to have enough guns to protect the US. Many of the guns are shipped overseas to support the different war efforts. The US has a very large supply of gun and they feel that it is necessary to protect the land and the people who live there.