There are neurological disorders caused by brain chemistry going haywire. The question is not "if" there is mental illness, but "what" is mental illness. I will use depression to describe an example talking about the layers and influences surrounding it.
Are you just going through a depressed phase, or do you have clinical depression? Big Pharma wants everyone to believe the depressed phase is clinical depression to sell more anti-depressants. However, depressed phases are merely people being a bit sad but overall functional. Clinical depression is basically not having the will to get out of bed at all or otherwise completely shutting a person down so they are not functional in society. Clinical depression is a severe issue with neurological brain chemistry in response to negative external stimuli, while a depressed phase is usually a fairly normal neurological response to simply being in a negative external situation.
There are many people, especially on social media, who see clinical depression as an easy excuse to stay in bed all day and not get fired for being a lazy shit. Medical doctors are sort of clueless when it comes to mental disorders and its 50/50 on whether you will get a good or bad psychologist to diagnose you. So medical doctors hand them out like candy along with half of all psychologists.
I mention the above because anti-depressants are a side treatment designed to get someone with clinical depression out of bed so they can go to therapy, work, meet with friends, essentially be a functional person. With the hope being that they will no longer need the drugs to function.
Its a giant clusterfuck though and there are so many points of external and internal, as anti-depressants are not perfect and we still don't have the best idea what is happening with neurological brain chemistry, failure that is is mind numbingly, literally and figuratively, difficult to properly treat anyone with clinical depression.
Just a reminder that Clinical depression is ONE OF THE MOST COMMON and least damaging mental illnesses to treat. Now imagine trying to treat a far more severe mental illness that is far more rarely diagnosed.
Is mental illness as common as Big Pharma and social media addicts like us to believe? Of course not. But legitimate mental illness is rising as the worlds economy crumbles and the weak generations of people who have not had to worry about anything for decades are forced to confront struggles that their forefathers would have built up mental defenses for in their youths.