Narcissists can't see other people's pain.

This means that they are more likely to steamroll forward on ideas that are good for them.

The steamroller is running over people who are screaming in pain… but the narcissist can't hear it.

Asked to justify what they're doing, they say "here's why it's good for me," completely oblivious to the downside for others.

Because the downside affects other people, and is literally invisible to them.