I'm wondering why are people with mental illness often disliked or alienated?

Tabboo: Historically, mentally ill patients were locked in asylums and crude forms of treatment were given. Nowadays, there is advanced medication to treat and get response and remission. Its hard for people to forget historical data about mental illness and people are always sensitive about the human mind and mental illness exposes them to face their own vulnerability, so to this date taboo exists.