We make promises that we have no intention of keeping. And we are reluctant to say anything uncomplimentary to a person's face out of a sense of delicacy. This is not to say that I am critical of such behaviour.
While it may be refreshing and certainly easier to face someone who will say what's on his/her mind, it is also nice to be surrounded by people who do (or seem to) care about your feelings and try and be diplomatic, even if only for form's sake.
And then there is something to be said about maintaining a formality in relationships
