Probably because I have a concrete notion of what "my privacy" entails, and I consciously don't place "things i'm typing in a search box to $company" in the "things $company shouldn't know about" box.

On top of all that, there's the functional aspect as well. DDG's results plain and simply suck, and using a search engine out of principle is purely an exercise in self flagellation if I can't accomplish the goal I set out to do.