Why do narcissists have such fragile egos?

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.

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.
- 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.
- Conducting experiments on people to test suspicions sounds like something a sociopath would do so maybe your friend is in good company /s
- What are you hoping to achieve? If you don't like this person and they make you feel tense, unworthy, and like you're walking on eggshells, you don't need to "prove" they're a narcissist. Even if they're just an asshole, the solution is the same. Stop talking to them and leave.

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.

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

They are deeply insecure, ashamed, alone, anxious, and afraid. They CANNOT be faced with others confirming their worst fear: that something is deeply and terribly wrong with them. They're basically emotionally disabled and can't empathize, work together as a team, or be genuine. They completely lack accountability ir responsibility. In close relationships, they suck the life out of people the most because it makes them feel better about themselves and they choose this tactic in these relationships because that person is least likely to leave them.

As a therapist, I've seen one, maybe two clinical narcissists. The one I remember best only came to therapy to get recognition for doing so but had NO insight into the nature of his issues.

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.

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".
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.
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

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