I'd say 2-3 main reasons.
1) Because Western Culture is kinda dominant at the moment. Even in countries like Japan Western culture plays a huge influnce. I mean look at a lot of Anime characters that just look... well white... due to anime being influenced by some western animation (Disney i believe.)
2) It's safe. What happens when San Andreas happens? Why it's racist cause a black guy is doing something negative. A lot of characters are Antiheroes. Also a number of videogame developers really aren't mature. Very few of them would what to do with a game set in an African or South American setting without resulting to offensive streotypes. Also you've got to worry that whatever game you made would be seen as a weird allegory to something weird going on right now. For example RE5 is going to be happening right near the Sudan.
3) HD Videogames tend to copy Holywood... plenty of games are even movie cash ins... so... that's more.
Everything is 99% white guys in media though. There is the same problems with Comics...
at least in Videogames all the black characters don't have black infront of their names.
