Could exposure to fertilizers be a factor?
https://www.scientificamerican.com/article/high-rates-of-sui...
https://www.scientificamerican.com/article/high-rates-of-sui...
But your point is valid. I have a cousin that farmed in the mid 20th century when they used chemicals like DDT on the farm. He is alive, but his body is falling apart. He is lucky that he switched to commercial/retail rather than trying to tough out a farmer’s life in his 80s.
The USA tends to assume everything is legal until there is evidence of harm. The EU tends to take a more precautionary stance and is more likely to claim harm with a lower barrier of proof.
Testing does happen, but the majority of it is done by the same corporations that have a financial interest in minimizing the negative results.
Round-up got caught in lawsuits mostly because they distribute to anyone, you think that amount of scrutiny is going toward every herbicide on this list? https://www.clemson.edu/cafls/research/weeds/management/herb... I'm not prone to fear mongering, but I can't imagine companies like Dow chemical and Monsanto putting much cash into long term mental health effects of exposure to any of their huge library of agro chemicals.
That's not even considering all of the other social and health care factors, as mentioned in the article. Honestly I'm surprised the rate is only twice as high as the average occupation.