I think that we are still in the infant stages of understanding mental health problems. We are far away from most people having a working understanding in the area. We can grasp a broken leg or even cancer. But psychosis and mania are beyond normal comprehension.
The prison figure is an example. Probably much closer to 50%, but they are untreated and undiagnosed.
Think this way: Atheists will not believe in something that is not tangibly, empirically proven. With that mindset they cannot have faith and they cannot accept the vagueries of mental illness. Yes were are making strides but many scientists do not even accept psychology as a science.
Mass media cannot sell viewers on matters of such intricacy. Here is an interesting idea, "intervention" I have never heard of it being used to get someone to get medical attention for a bleeding wound. But it is standard for mental illness.