Why is it that all narcissists have so many personality traits in common?

Because they are narcissists.

Answer

To be a Narcissist in the strictest sense, one would have to fit five of the eight criteria for having the Narcissistic personality disorder. I believe this is explained in depth in the very first FAQ "What is a Narcissist?"

Even then some of their quirks and nuances are chillingly similar. The way they look or don't look at you, The facial expressions. The phrases they use. Mannerisms, etc. Even the way they dress.

What is odd to note is that anyone suffering from any personality disorder has so many traits in common with others suffering the same disorder. Yet any of one "healthy" individual has traits unique from others.