How can psychologists define 'mental health' when we know quite little about complexities of human life? Erich Fromm in his The Sane Society (1955) argued that mental health should not be defined merely in terms of the person's harmony with his/her society. He believed that a society, as a whole, could be mentally ill. He said that all humans share a common human nature, with which some cultures are not in accordance.

Maybe that's the cause of the collapse of some civilizations in the past.

One big problem here is the complexity of human brain and human life which make predictions difficult. For example, in the 60s and the 70s a sexual revolution occurred in western countries. Psychologists today consider some sexual behaviors normal/healthy which were traditionally discouraged. But in comparison with the age of human civilization on earth, these changes in norms are very recent experiments, about the results of which we may not be able to be sure.

We can see the same problem in economics, which is, in some sense, based on psychology. As an example, since the 80s neoliberalism has been the predominant economic theory in western countries, but now some big names in economics state that it is a failed experiment, causing many problems like deep inequality and the rise of populism.
Although you are correct in thinking, mental health is not defined merely by what others feel, it is a lot more personal. If I am understanding your question correctly, you believe that mental health is based on predictions. While predictions are necessary for defining a lesser known disorder, ones such as major depressive disorder and bipolar disorder are quite common and have been so for long enough that diagnosing them is relatively easy. In addition, it can be widely agreed upon that behavior such as suicide and self-harm are abnormal. A persistent change in mood is another indicator of a cognitive error. Lastly, hallucinations and delusions are also clearly indicative of an underlying problem. All of these symptoms must be present without the use of drugs, obviously. Any other behavior or thought pattern that interferes with daily life and/or is societally unacceptable should be a reason for alarm. These symptoms are clear and require no similar cases to diagnose an "issue with mental health" Aside from common, well-known disorders, evidence of other cases must be present in order to label such as a specific disorder. What I cannot prove are the disorders that are few and far between, and seem highly implausible. Regardless, if one seems "different" for an extended period of time and does not seem to be improving, it would be wise to seek help. Hope this makes sense, it was a fun question to answer;)