A couple reasons.
- Most of these games you're complaining about are made in America. Where's the first place the developers are going to look for voice actors? You guessed it- America! Is 2K Boston supposed to fly someone out to Glasgow or Nice or Cape Town for voice auditions? That costs money, and in case you haven't noticed, the industry as a whole is having a bit of trouble with cash these days. Besides, has anyone here played Far Cry 2? I've yet to see a character who speaks in American English, and half the enemies speak Afrikkans. And what about Onimusha 3? It has Jean Reno! How much more French can you get?!
- Putting characters with "foreign" accents in games is almost always complained about by citizens who live where the character supposedly hails from. For example, I remember several Australians griping that the accent of TF2's Sniper was "generic" and somehow insulting. And of course, there's people who'll whine if a character comes with a regional accent that they don't like. (Really, we Americans are guilty of that with our own, too- New York accents seem to be just peachy, but Southern or New England accents? Almost never heard, except as parody.)
Should I start complaining about the lack of Americans in Doctor Who or Red Dwarf?