The trouble with being able to predict the future is that the existence of such an ability would imply that the path the future will follow is somehow set, which further implies that you can't use these predictions in any attempt to change the outcome because, in a sense, said outcome has already happened. If it were otherwise, it would be impossible to observe the future. If what psychics "see" is a possible future, it becomes, impossible to verify that the ability exists and the person is not just guessing or hallucinating. Think about it - in that scenario, the very act of observing the future changes it.