I really hate when people do that.

Anyway, I think ignorance can play a huge role. Most people don't bother to learn about mental illnesses, and usually rely on media to get their information. However, the media usually portrays the mentally ill in a bad light, especially when it comes to more stigmatized disorders like schizophrenia. Hell, when a school shooting happens, it's usually blamed on a mental illness, and I had a family member use that to prove that psychotic people are violent.

Thankfully my university does have a mental illness awareness but I'm not sure how affective it is.