It also benefits the appearance of the final product. If everything was fully detailed the image would be too saturated, the eye wouldn't find any details appealing. In this image, key areas are more detailed; weapons and some armour. This way they stand out more. While I agree that these guys are expected to do a lot of images at a time, as this is loading screen art, the artist would be expected to spend a lot of time on it. Also, one day is a lot of time for one painting; if you're a professional and talented artist.