In your experience, why do mentally ill people not seek treatment?
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.
It's not always irrational to refuse, but a total absence of insight into illness precludes a rational decision.
I am speaking from experience as a depressive person (not actively sick right now): I know that I do not think objectively when I am ill, but I only know this when I am not ill.
Being depressed is terrible. I cannot imagine what hallucinations must be.