Question

Why is it that some people believe that metal illnesses are not a thing?

Answer 1

They base it on experiences with people they have had in the past

Answer 2

People who don't believe in mental illnesses think the person with the mental illness is just lying about it. people may not believe in them because it's not like a physical illness where you see the symptoms or you clearly see that the person is sick. it's just the matter of not seeing physical proof of the mental illness.