This is more of a deeper philosophical question.

Why is Narcissism treated like a disease?

It's not some biological organism or a bacteria that people can catch. From what I've learned dealing with the numerous narcissists in my life, I've noticed one common thing. They play by the same rules in their own individual way. They're somewhat functional adults some of who have jobs and families, not lepers with marks everyone avoids.

My narcissistic mother didn't catch a bug suddenly when I was twelve. She had been depriving me of emotional support and anything she could legally get away with since she was born. She was smoking marijuana around a four year old and his infant brother, neglecting to potty train the two. She didn't treat a teenage boy like trash simply because of some infection. She already wounded him before he turned eight, multiple times.

My narcissistic ex didn't steal my guitar because she suddenly got a virus. She did worse to other men long before she met me. Accused her ex of stealing from her and used everyone around her.

My former neighbour didn't single me out because of a medical condition. He treated everyone like lower class trash long before I ever knew him.

I don't see it as a simply psychological disorder either. I see it as a choice. An uninformed one. But one nonetheless. They had some level of awareness to promise to look out only for themselves and hurt everyone else. They had a choice to be decent people. At some point, they could have turned back. It was the uninformed choice they had chosen one too many times that became a habit. My NMom chose to do what she did. Because she had made the decision as a child to be that way. They could have stopped. And they could, if they were willing to break the oath. But they swore it and it's the only one they keep.

But the key is that this is an uninformed choice. A promise they made to themselves without realising what it would make them later. It became their oath. The one oath they are unwilling to, and incapable of, breaking. They treated it like a medical condition and society treats it like that. This becomes an excuse.

I thought back long and hard about those three and the three of the near dozen other narcissists I've met. They didn't play by the same rules as written. They played by the rules in their own ways out of something they sought.

They made an uninformed choice, and it became a promise. One that voids all future promises to others that conflict with this oath.