That said, it's also worth talking about which things aren't derived from changing standards, or at least recent ones. The perception of something like schizophrenia or suicide may differ between cultures, but both have been pathologized in the US and Europe since pre-industrial times. Less dramatically, any psychological condition which leaves a person largely housebound (e.g. extreme depression or some presentations of OCD) has been pathological since at least the end of the Victorian Era.
To the extent that we see a rise in suicide rates, debilitating depression and anxiety, and patterns like hikikomori and its international analogues, it really does seem like something has changed about behavior as well as society.
The diagnosis for ADHD specifies multiple areas impaired. Just flunking class or getting fired won't do it. Unless we think maintaining basic friendships or relationships, suppressing destructive impulses, defeating procrastination, avoiding chronic disorganization (extreme clutter), staying away from addictive substances, etc., has also become significantly more sophisticated, I'd think the aspects that affect those types of activities would stand alone as an indicator.
There have been epithets/cliches for many, many years that come down to pathologically spaced out, lazy, foolish to the point of not being "normal"; and we've had "minimal brain damage" and other pre-ADHD executive function impairment diagnoses for a long time as well. It's not just a modern thing.
From the American Psychiatric Association:
"Mental illnesses are health conditions involving changes in emotion, thinking or behavior (or a combination of these). Mental illnesses are associated with distress and/or problems functioning in social, work or family activities."
https://www.psychiatry.org/patients-families/what-is-mental-...
For some mental illnesses, this pretty much does amount to society defining the condition. That doesn't mean they aren't real, just that diagnostic standards including "interferes with functioning" will necessarily shift as the meaning of everyday functioning varies.
The classic example is someone with poor cognitive control meeting the ADHD standard of "difficulty in at least two settings". Trouble maintaining a home, but not socially or professionally, doesn't suffice, but trouble in home and education does. So the same person could be mentally ill in a place with compulsory education through 18, but not mentally ill in a place where they drop out of school earlier and find a job that works for them.
I have seen enough of this up close to know that it isn't society's construct, that many sufferers are objectively having problems.
Maybe not all cases and conditions are like this. But it doesn't do anyone favors to handwave some of those serious conditions away.
For an interesting tangentially-related thought, try asking people what the definition of "healthy" is for a human. Mental health is the hardest part to define out of all health.
I would argue that the average human being doesn't consider these things easy to acquire. For society it COULD be easy to distribute these things if we decided that was a goal of ours, but we haven't gotten there yet. I would be very interested in seeing the ideas (or problems?) invented by children born into a world where this was true, but unless things get very interesting very quickly, I doubt we'll get the chance.
Without the immense pressure to conform to insane, abusive and destructive standards in combination with living in a harmful environment with poisonous "food" and "medicine" in competition with others who are likewise fighting to keep their head above water; I suspect most mental illness would disappear by itself.
Treating mental illness like some kind of natural law which can't be avoided, only stops us from finding the real causes and progressing towards a more humane society.
However, when it comes to an individual person being stuck in one of those states... What you are saying winds up feeling a tad ridiculous. For example, if presented with someone who has a delusional belief in something objectively, demonstrably false on a factual basis, to the point where acting on their belief makes them a danger to themselves or others ... It is not the case that a different society will make those delusions into true facts. What they need in that moment is probably medication. I have seen people for whom that has definitely been the case, and they show improvement with treatment.
I'm not saying a more humane society would cure existing mental illness, though I'm sure it would help.
The point is that most mental illness is a product of the society we live in, and the long term solution is fixing that rather than medicating the symptoms.
Blaming chaos or genetics, that's how you over-simplify.
Now you seem to be suggesting the conditions are expanding, as if invented, to serve a profit machine.
This all suggests to me that your real opinion is that the illnesses don't exist.