It's not that the gameplay is bad, its that the concept is boring, here we have (insert country / alien race / mutation / supervillain name here) attacking America like many hundreds of movies and games before, nothing has changed, the locales are the same and the enemies are similar to the other enemies you played against last time. Notably, the enemies tend to invade popular places in America rather then unique places not used often like Miami or Atlanta, heck, even south America like Mexico. It always takes place in the biggest cities. Quite frankly it is kind of boring for those living outside of the US, its still fun, so we still buy the game, but its hard to relate when you have no connection and / or interest in the country you are playing as.
For example, GTA yet again taking place in America is no suprise, but to be honest, it's getting kind of dry, I'd like to see GTA London but we all know that won't happen. Heck, I'd like to see GTA Russia involving the Russian Mafia, but we all know that won't happen either. I'd also like to see a game whereby a criminal escapes North America after comitting a crime and goes to the south where he tries to rebuild his life or something, but again, that won't happen.
Its not that people dislike America, its just that every game seems to have the reoccuring theme of America's involvement, scratch that, North America's involvement, which begs the question, "If it's America getting invaded and not us, why are we saving them again?"