We have so ingrained in our minds that a person's value to society is defined by his/her economic contribution in the traditional sense (i.e. via a job or business ownership), that we assume that a person who does not work has no value by definition. In fact, they are even morally deficient. So, how can they possibly be of a quality/character as to contribute anything to society via arts, ideas, or otherwise?
It's a sneaky bit of circular logic that, for many, argues against a BI.