Why do narcissists have such fragile egos?

Because narcissism is, at the core, intolerance of anything but the self.

The fragility is part of the narcissism. You don't get one without the other. In the end, narcissism is an extreme projection of insecurity, which is why "narcissistic rage" is a thing.

Isn't the whole point of being narcissist?

Their ego and their self esteem are so fragile and low that they have to be mean to protect it at all costs.

There's a little bit of pleasure in doing narcissist things. But the main point is to protect the fragile self.

Cause they only focus on themselves? If that gets attacked, it's an attack on their only asset. When your focus lies on a spectrum of interests other than yourself, you keep an overview and your overall self worth. I dunno.

Hmmm... That gives me an idea.
I know someone who might be a narcissist. Maybe I could exploit this vulnerability to expose him. You know, just to test my suspicions. Could just say something that mildly challenges his ego and see how he responds. If he menages to keep his cool like a normal person, then my hypothesis was wrong and I don't have to be so suspicious of him any more.
- Careful. Skillful Narcissists can be quite adept at keeping their cool in public (to maintain their positive public image) while simultaneously planning their complete and total revenge on you - smear campaign, sabotaging relationships, setting up traps, lobbing false accusations, messing with your employment, messing with your spouse, vexatious lawsuits, the works.
  If you play with a Narcissist, be prepared to go ALL THE WAY when you dance with them. Complete and total WARFARE. One party survives and conquers, the other is absolutely DESTROYED. It’s how they do things.
  Really, unless you have some compelling reason to do so, the only winning move is not to play.
- I'm with Photuris, it's a much more complicated game than you might expect, and they've been playing it their whole lives.
  Rule #1 is to preserve your image at all times. Which means not flipping out in public.
  Rule #2 is that everything is someone else's fault. Which means if you call them out in public, they will cry and ask why you are so mean to them. Instant sympathy, and you look like the asshole, and assuming you yourself are not a narcissist, you'll probably feel like an asshole too.
  Most likely, if they really are a narcissist, you'll try your test, think they aren't, and look really bad in front of everyone.

I feel like it's OK to demonize narcissism in ways that would be unacceptable ableism if you were talking about other neurodivergents or personality disorders.

It's just as blanket and vague as calling someone an asshole.

It's become such a pop-psych phenomena, I think it's time for clinical people to abandon the term to pop culture. When a diagnosis becomes a common insult, it's time for a new term.

I'm sick to fucking death of the word being tossed at anyone acting an ass. My mother had NPD, only person I've been close to that certainly had it. Got on r/raisedbynarcissists and it was a bunch of whiny little bitches whose parents were merely selfish, and sometime only occasionally.
The reason is simple: all the main symptoms of narcissistic personality disorder involve behavior which is harmful to others.
In this, it is different from most forms of neurodivergence. Like psychopathy, it is a kind of antisocial personality disorder, in the sense that people who have it are harmful to society. Autism, ADHD, bipolar, BPD and even schizophrenia do not necessarily cause the sufferer to become harmful to others - in fact, the person afflicted with these conditions often harms themselves on an attempt not to inconvenience others, and only harms others unintentionally, while the narcissist does so intentionally and deliberately. People don't demonize narcissists. Narcissism demonizes narcissists, and people recognize that.
Narcissism is not neurodivergence, it is a personality disorder. There is a great difference between them. But to your point, I think that the proper term is "Narcissistic Personality Disorder".
Tbh I think creating new terms just muddles medical records over time because now when you go into a field you have to learn not just a disorder, but also everything it's been called throughout the last 50-100 years in case you're going through the records of a patient older that 50-70 years old. I think the public needs to be better educated on these things. And when they keep doing it anyway that just kinda is what it is. That said there's definitely room for phasing out diagnoses and replacing them with entirely new ones that differentiate or combine what we currently know as our knowledge of them evolves.

How dare you imply my ego is fragile?! Don't question me!

Are you making fun of me?!
- Sorry for not being perfect enough for you! I don't know why you say such horrible things to me, I was only trying to help!

A narcissist sees the rest of the world as inferior to them their perception of reality is all derivative of this opinion. When confronted with objective reality disputing that they don't just react to the singular confrontation. Because their psuedo superiority underlines everything they think about any evidence they are wrong begins to dismantle every opinion they have. Now being forced to consider the entire way they perceive reality is flawed its easier to get angry and confrontational instead of reevaluating their worldview

My experience with a clinically diagnosed narcissist, which is very rare considering, was with an ex I spent/wasted seven years of my life with, and subtly different from what you said. She saw everyone else as superior but unjustly so. Everyone that wasn’t serving her interests was an enemy. The insecurities of feeling inferior to everybody manifested in a projection of superiority but under it all it was just a mask to protect her fragile ego. She would do anything and everything, in the moment of her current environment to get what she wanted. She only cared about anyone in so much as they validated her desire of inflated self worth, her own children included. There was a sliver of hope after a year of therapy but as real as the temporary empathetic soul briefly emerged all it took was a light switch moment and suddenly, like a light switch, that just became another momentary blip to then be used as an additional tool in her toolbox to socially destroy anyone that wronged her. Even after grey rocking her for over two years, following our seven year relationship, did I start getting left alone (luckily her kids weren’t mine, still feel for them but they are taking a master class in manipulation and strategic mood swings). I’m sure she got distracted with her next victim and I no longer fed her need for control and ego building enough so she lost focus but it was the most miserable decade of my forty odd years on this earth. I love her, or who I thought she was anyway, it still hurts thinking back but it was never really her, who I thought she was didn’t exist.

I expect the causation is the other way around. Someone with a fragile ego may become narcissistic to protect themselves.

Jesus. I wonder if that's what happened to my mother. Never understood how her two siblings were so smart and successful and she was, frankly, a dipshit. Being the youngest and clearly the dumbest must have had an impact.