I think it's mostly cowardice, personally. People don't want to risk putting their own choices into a game based entirely on choices, just in case they aren't as good. It's better to use someone else's decisions than risk your own pride.
Then you have ignorance. A lot of people don't know how to fill the gaps, and WotC has never bothered teaching them how. Any rules they did get are rules of thumb and aren't something to use without thought (like CR), so people complain for reason 1 again.