Oi, all of you jumping to defend the Russians' image in video games: stop it. I'm Russian and it's totally fine, we don't need a champion. Guess it's because we aren't nearly as PC as the Americans, so we don't feel like a Russian villain is a personal affront to all of us.
The Arabs: well, it's kind of obvious, isn't it? Some of the real-world bad guys of the 21st century happened to be Arab, America waged war on two separate Arab-culture countries and is likely to go to war with a third one anytime soon... An average person instantly associates a bearded Muslim guy with 9/11 and the London bombings, so they make for good stock villains. Besides, it's not like they're the target audience of these games, so predominantly American game developers and publishers don't have to worry about losing a market.
That is not to say I support demonizing Arabs in fiction, but you asked "why?", not "what do you think about it?".