Because it's indeed difficult to portray, in any meaningful depth, another being, his appearance, speech, mentality, without to some degree, and often for quite trifling cause, offending him. The truth seems to be the nobody likes to see himself described as he is, or cares to see exactly set down what he said and did. Well, even i can understand that - because i don't like it myself when I am the sitter not the portraitist: the frailty of egos- and the more accurate the strokes, the greater the resentment.