The reason is the narcissist's enormous life-long, irreplaceable and indispensable emotional investment in his disorder. It serves two critical functions, which together maintain the precariously balanced house of cards called the narcissist's personality.
Most narcissists reject the notion or diagnosis that they are mentally disturbed.
Absent powers of introspection and a total lack of self-awareness are part and parcel of the disorder. Pathological narcissism is founded on alloplastic defences - the firm conviction that the world or others are to blame for one's behaviour. The narcissist firmly believes that people around him should be held responsible for his reactions or have triggered them. But that is not to say that the narcissist does not experience his disorder. But he re-interprets this experience.