I suggest that our definition of mental illness is more fashion than science.
I did not mean to suggest that mental illness is fashion but I am certain that the changes in definitions as to what is and what is not mental illness is not hard ass science.
Look … at one time homosexuality, gender dysphoria, and alcoholism were considered either diseases or crimes. We changed that, not by discovery but by changes in law and perception.
Take alcoholism, for example. It always had a very negative stigma, as if those who suffer from it had a choice. Because of the type of behaviour that often comes with it, such as aggressive actions, it was almost considered a crime. Now, thanks to new thinking, this has changed and it is now seen by many as a disease. Instead of judgement, alcoholics can receive help from places like Enterhealth (https://enterhealth.com/). This is a change for the better.
We have also “discovered ” diseases that but called something else or may not have even existed .. eg post traumatic stress disorder and gluten sensitivity.
Look at the differences in how we treat obesity (my problem) vs homosexuality. There is a great deal more evidence that obesity is genetic then that homosexuality is genetic, yet today we celebrate homosexuality as a normal state and consider obesity a matter of choice. Imagine what happens if (or when) we acquire equal opportunity for fat folks on track teams.