In many cases they genuinely want help but don't know what to do. Some of these people use manipulation because it's a part of their culture. They may not even realize that it's a problem. Many people genuinely believe that everyone manipulates others and are just doing what they think is appropriate. Until recently mental health services have not been good at understanding this distinction.
Psychiatry is a relatively young science and there is still much to be learned.
The process of learning, like the process of helping is always hindered by deceit however and clients in contact with mental health services generally do better by being honest in their dealings with professionals. If you genuinely want help with your problems it's important to trust clinicians to do what's right. Given the chance they generally will although giving you what you need isn't always the same as giving you what you want.