As a general rule of thumb, the more seriously ill someone is, the less likely they are to realize it. Like every "rule of thumb" there are many exceptions, but it's true enough.

The other factor is that many medications for the treatment of psychotic disorders (psychotic meaning hallucinations, delusions, and disorganized thoughts - NOT meaning wanton violence) tend to have unpleasant side-effects. If you are convinced that you are not ill, you wouldn't want to take them either.