Theres one obvious and glaring reason why many games take place in the United States:

Many of the world's Game Developers are in the United States. These Developers quite often have experience with the locales they portray in their games and are familiar with them. It also depends heavily on the tone and aesthetics the developers want to portray in their games. Take a look at Half Life 2. Valve wanted a place that had a long history of oppression, depression, wear-and-tare and could provide that all-too awesome old-age meets new-age European city flavor, so they went with Eastern Europe (assumed either the Baltic States or Ukraine). The aesthetics of a region play heavily into where developers base their games and the USA tends to meet those aesthetic requirements quite often simply because there are a lot of varied aesthetic divisions.

Its not just because people like to blow up the USA, but that everyone likes to blast the big-guys; take boss-battles for instance.

Look out People's Republic of China. Game developers are setting their sights on you next, possibly within the next few years.

so you want to attack foreign country?

hell yeah just sumwhere new like spain or italy for a change, or a gta style game in tokyo.