Narcissists don't see that people have vulnerabilities because they are incapable of seeing their own. They exploit people's vulnerabilities but it is always debated whether Narcissists know what they are doing or not. It is the nature of their disorder. The superficiality, same thing. They are so self-centered because again it is the nature of their disorder that it is very hard for them to connect with someone on a deeper level.

A lack of empathy is seen by most as a hallmark of NPD. Again, it is the nature of their disorder. It's not that they don't care exactly, it's more like they are totally unaware of anyone's feelings because they are so wrapped up in themselves.

I've known quite a few narcissists, and only one of them was a sadist. But they are totally oblivious to how their actions affect others with few exceptions(that sadistic malignant Narcissist that is deceased now), again there is the NPD style of lacking empathy.

As to why nobody tells a Narcissist what you mentioned, frankly many people are intimidated by Narcissists or conversely some people are familiar with Narcissists and know better than to try and "get through" to them. Then of course there are the slew of people who try to "get through" to a Narcissist that is their romantic partner, parent, etc and they end up emotionally devastated because no matter what they say the Narcissist is oblivious and cannot see how they are hurting their children/partners/etc. Most Narcissists are not very self-aware. 99% of the time trying to talk some "sense" into a Narcissist is like talking to a brick wall. You are not going to get anywhere. Most people don't know how to handle Narcissists, I do but I realize most just don't because they haven't had as much experience with them as I have... that and I am not blindsided by emotions that most people have, so I can look at a Narcissist objectively. That helps a lot.