A rational decision is weighing the pros and cons. If you are blind to your illness (and the ramifications, for example having killed a family member you care about because you believed them to be a demon), then how can you weigh the pros of having those delusions resolve vs the cons of weight gain and potential diabetes?

It's not always irrational to refuse, but a total absence of insight into illness precludes a rational decision.

There is no “rational” decision when your mind does not work correctly. Subjectively it is but it is objectively both wrong (because false) and imprudent (because data is discarded which should not be).

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.