DD is not the only Fate that can be farmed. It just happens to be high enough level to weed out a lot of stupid and people mean enough to allow that one to be farmed. There are lots of fates very similar to it (often turn in types) that can be farmed for massive exp but people constantly turn in.

Most common reason? "I can play however I want"... I see that as being a selfish dbag.

Many people don't know better but after being told if you continue to do it you are a jerk.