Its a good question and I am unsure why I would get a downvote a it delves into realms of philosophy. Ah I just realised its the offended manager. Anyway to answer the question, I think at a core level I just always felt that I should be loyal to my manager, and if I saw that after my sacrifice, that I was unappreciated, then I would leave, as my work was not for payment but to help people who needed it.
Once decided to leave, I felt as a final mark of respect I would both leave quietly, and more importantly I did not owe them the feedback, which would ofcourse help.
It sort of should make sense. I am often confused why people who decide to quit a company that has wronged them, would voluntarily provide feedback. I feel they only do it to vent, but control of ones emotions is an act of discipline one should not shirk in professional settings.
