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

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

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.

This answer would be true