Look at the effort they put into Art that has absolutely NOTHING to do with the game. Look at the incredibly detailed and well modeled intro videos to the expansions and games...
I mean, look at the new SC2 expansion video, that probably took more people longer to make than putting those same people putting that time into something that's actually constructive that actually makes it into the game itself.
I understand they are "just artists" but there are many things that require art that we've asked for that are on a back burner cause it just isn't what blizzard "wants" to do.