Not the parent, and I don't mean this sarcastically, but isn't that akin to saying "Only 5-10% of people fracture a bone, so why bother treating it. It's just a standard part of human existence."?
Also poor eyesight, color blindness, partial deafness, &c. The reality all of these diseases/disorders are readily and inexpensively treated and not an issue in modern society.
There is a wide difference in the prevalence, severity of condition and extremity of treatment in some of these conditions.
Poor eyesight afflicts a large portion of the population, but the severity of the issue and the extremity of the treatment are not high. It doesn't impact your ability to maintain social relationships, for instance, and the treatment is very well understood and has low/no side effects.
Color blindness, partial deafness etc., are readily and inexpensively treated, but they do not afflict 5-10% of the young and healthy population.
The issue comes in when we start saying 5-10% of the population has an extreme condition (ie: impacts ability to maintain work, social relationships etc.) with a relatively severe (ie: fairly large % of users experiencing side effects) treatment necessary. When you put that in the context of our massively volatile understanding of mental illness over the last 50 years, as well as the massive incentives for pharmaceutical companies to push a specific narrative, it is difficult for me not to see a red flag.
Off hand I have no idea, but it's also the severity aspect here that is different. Color blind people do not have problems with executive function, maintaining social relationships, depression etc.
"...massively volatile understanding of mental illness over the last 50 years...": The sort of volatility you undertake on a path toward drastic improvement.
Don't false equivocate. If we were to open more mines than there were in 1900 we may well see more broken legs. However we're far better at treating a broken leg.
Though it is not at all clear that "mental health" is deteriorating. It may be that some forms of mental health issues are more prevalent today (eg. anxiety and depression). But that is no where near the whole picture.
An entire generation of people were traumatized by WW1 and WW2, and by the generation of parents who through that traumatization, traumatized their children.
Extreme disorders of the personality, PTSD, etc. simply were seen as normal then because they were so overwhelmingly common. PTSD didnt exist as a diagnosis till 70s, and yet it would characterize a generation of people under war.
If I were to speculate, I would say that we are overwhelmingly better in our general mental health.
That we recognise it as an issue suggest a profound advancement.