Because what any human wants is a moving target. As soon as someone else delivers exactly what you ask for, you will be disappointed unless you suddenly stop changing. Think of the dilemma of eating something you know you shouldn't. Whatever you decide, as soon as anyone (AI or human) takes away your freedom to change your mind, you will likely rebel furiously. Human freedom is a huge value that any FAI of any description will be unable to deliver until we are no longer free agents.