Yes, treatment is vastly more important than the actual diagnosis! But having a diagnosis can help alert both the individual and other professionals involved as to what symptoms are likely present. Not that they all will be - but then again, even non-brain related health issues rarely show all the same symptoms!

The reason why the mental health diagnosis labels change is because the brain is a much more complicated organ than the nose, stomach, etc. We can't visually SEE most problems with it, and technology isn't all the way developed yet. Even once they're fully developed it will still take a long time to properly understand them.

Science is a process, but it does the best it can. While it's not perfect, it keeps working to improve. Maybe someday they will have something more concrete to help in diagnosing the various disorders affecting the brain. Until then - trial and error, hyposthesis and revise!

And even the treatment of a cold or flu varies from individuals, so why wouldn't it be the same for mental health? The individual and the professional have to work together to find out what works. With simpler ailments it's just easier to find the most common helpful things.