Because those people usually tend to harm others without regret. Its an instinct that most humans do not like people who appear arrogant because we are social creatures. Confidence is great and attractive. Being aloof is harmful to society

That part I swear

Also, confidence doesn't always equal high self esteem. If you practice something enough, you build your confidence in your skills. You can still feel like you are not very special overall, but if you are in an element that uses the skills you've practiced, you can appear highly confident simply because you are in your element. I think people project & assume too much about what other people feel, which kills curiosity and keeps everyone trapped in false assumptions and their own insecurities.