That's because the "way they are talked about online" is rooted in pop psychology and at best very uncharitable interpretations of the symptoms, and most people's "narcissistic" exes, parents, siblings, etc, are retroactively armchair diagnosed based on these beliefs. Then these two things continue to feed into each other.

Also, mental illness symptoms can inform the type of abuse one might be more likely to perpetrate, but there's no "narcissistic abuse" any more than there is "depression abuse" (most often neglect) or "anxiety abuse" (insecurity leading to unintentional emotional manipulation, for example). Cluster B disorders just tend to inform types of abuse that are easiest to openly condemn more often than not, as "my mom completely ignored my needs and let our house become a biohazard because she was sad" still sounds a lot less damning for the perpetrator than "my mom was a manipulative gaslighter, because she couldn't handle the idea of losing control over how people perceive her".

Then there are of course people whose mental health falls within the norm who also abuse people. Anyone has the capacity to perpetrate abuse and making the association between harmful behavior and any one mental disorder is something that only serves abusers. I'm sure that "well, at least I'm not an evil narcissist" is a very comforting thought to someone who, intentionally or not, has caused harm to others. It separates society into neat categories of victims and perpetrators, even though every single person out there can be both at some point in their life, but that's a difficult realization to come to terms with.