The article I link to is specifically related to bipolar but the sentiment is true of film and TV representations of all mental health disorders.
The why, though, is easy.
People who don’t suffer from mental illness just don’t understand it. That’s true of most illnesses of course, mental or otherwise, but mental illness isn’t afforded the same respect as other illnesses. It just isn’t.
Despite all the science pointing to mental illness being a combination of biological, genetic and environmental causes, just like many other illnesses, you’re more likely to be blamed for your mental illness. Many people don’t see you as “ill” and assume you should just be able to think yourself better.
The short supply of empathy for these conditions is why so many sufferers find themselves living on the edges of society, either physically or emotionally.
Sadly such opinions are often prevalent even in medical circles, so it’s not surprising TV and film gets it wrong too.