Because they're really no different from American developers. They're in it for the business. They want to make money, and the biggest market for gaming is still here (here where I live, USA that is).
Plus, unlike Japan, European gamers really don't have much of a distinction or preference in video games outside of what Americans prefer and like. You guys like Halo, Fallout, Mass Effect, Gears, God of War just as much as we do.
Japan on the other hand has a longer history of developing video games, hell, without them, gaming would have never become what it has. Plus, they just like being different. Taking an invention, name, whatever, and making it their own.
Unfortunately you Europeans have become way to westernized, you guys think way too much like us, at least when it comes to business and entertainment....=P