Because there’s nothing to prevent it.

Well, it would be a bit unfair to talk about “European” games and “Asian” games, and on the other side “USA-made” games.
There are three countries in North America, we should at least include the two others.

Fallout, Wasteland, The Elder Scrolls.
Also smaller games come to my mind, like Child of Light or South Park: The Stick Of Truth which are made by Ubisoft Montreal.