Reason 1.
Liberals are the ones who responded to poor conditions to institutions by letting the mentally ill out into the streets with a prayer, instead of improving the institutions. The same is true of their tendency to open the jails and hope for the best of human nature. We need to rebuild the 150K mental hospital beds we had in the 1950s, put involuntary mental incarceration laws back on the books and put a lot of the “homeless” there. But the liberals are the ones who let them out in the 1960s in the first place. Doing what needs to be done is an admission the original action was wrong.
Reason 2
Liberals want the majority of the population to be a mass to be manipulated, with themselves as the elite. Plato versus Aristotle is the liberal-conservative divide today.
It is easier to consider your opposition crazy or stupid than argue facts, figures and rationality. They want masses of social workers policing the actions, behavior and even thoughts of everyone - not forcibly medicating and restraining if necessary the 1-3% of the population that is truly, severely mentally ill. Better to control men by pathologizing them in the guise of mental health than call it a police state. And the excuse is the mentally ill on the streets.
Reason 3
If nothing is anyone’s fault but society’s fault, how then does one explain evil? Excusing evil with a medical diagnosis is one way to explain it, and offers those who deny the existence of personal accountability a way to “cure” it. With, of course, more nanny-statism.
Reason 4
There is more money in making everything other than the perfect personality a mental disorder, instead of spending the resources on the small minority of mentally ill. More money = more power and more jobs for the elite.
Reason 5
To borrow from Atlas Shrugs, if you don’t have lots of mental disorders, you don’t have a way to control people. Parents whose children are labelled as ADD or opposition defiant can be forced into compliance with state supervision or loss of freedoms via a mental health diagnosis and threats of children taken away for failure to “treat” the diagnosis.
Reason 6
You can do a lot of political arm twisting in the guise of medicine. The Soviets invented “sluggish schizophrenia” to lock up those who didn’t see Soviet Russia as paradise. You could lock up and torture political dissidents without officially jailing them.
And there are plenty of liberals who think that conservatives are crazy. And I’ve come across psych surveys seeking to build up a mental disorder to fit “authoritarian, domineering, closed minded” into a mental disorder. Once invented, guns can be taken away in accordance with the mental illness controls on owning weapons, without overtly violating the Second Amendment. One simply declares are conservatives crazy, then takes them.