It's not about getting something for cheap. It's about being able to make derivations and incrementally tweak things. It's about that not having any base tweak makes it quite hard for both Artists and Coders. It's about that there's many ways that things could be tweaked that would be fun, but if you're beset by a wish to do that you're gonna spend a year creating art for a total conversion. It's about that culture in games is not code, it's assets as well, and if the assets aren't released, they're lost and never to be derivable of, because of the perpetual copyright crap.