Because they are ableist
I do have issues with the DSM, where did I say I excuse their use of it?

Wait, so are you saying it's ableist to use the word "narcissist" outside of a medical context, or that the DSM choosing that word for a medical diagnosis is ableist?

We don't have to use it at all, if someone mean "abuser" or "bad person" they can use those words

Those words mean different things. If I call someone an "abuser" and/or "bad person" I am not implying they have narcissistic tendencies or issues. Likewise when I call someone (who has not been diagnosed with the personality disorder) a narcissist, I am not calling them an abuser or even necessarily a bad person. It is also a matter of scope and specificity, with there being millions of ways that someone can be a "bad person" and narcissism can be one of the contributing factors. Just because a word is being used as an official diagnostic term does not suddenly make it ableist to use it in other ways or in other contexts.

Look, I am a disabled person and I have a personality disorder. Specifically, I have Avoidant Personality Disorder. It would be ridiculous of me or anyone else to say that we should stop using the word "avoidant" outside of a psychological diagnostic context. There is nothing ableist about referring to certain behaviors (or people who exhibit those behaviors) as being "avoidant" when we are talking about issues outside of the tiny narrow slice of the population who (like me) are diagnosed with the actual personality disorder that carries that name. "Avoidant" is a regular word that can be used in all sorts of contexts without being even slightly ableist. So too with narcissism or narcissist.

I am all for consciously and intentionally changing our vocabulary so as to remove the common ableism from it. I've even argued in the past about how we should not use insults like "moron" when it was more common to do so here. But it is also possible to go overboard and label the use of certain words as ableist when they are not. Sometimes that overzealousness, even if it comes from a good place, can do more harm than it does help, as far as encouraging people to recognize the casual ableism in their speech, when you apply it in genuinely inappropriate places.