They currently have to pass those tests which is why they're not morons. But if you decide to make it possible to have any of these mentally disabled people then I'm sure someone will find a way to turn it into a lawsuit if we exclude them. e.g. they had a test to hire police that turned out to have 'disparate impact'. The questions were simple things like: if a shift ends at 11 PM, the policeman shows up on site at 10.45 PM and stays there till 1 AM, how many hours of overtime did they work?

But some people couldn't answer them and so they got paid out. When the system is opaque we can preserve competence. If we have to "make reasonable accommodations" and "prevent disparate impact" we're going to be in trouble. Better to keep it quiet.

The government that had to pay out because people couldn't answer that question was also keen on not paying out lawsuits. But they had to nonetheless.