Why Do We Criticise Main Characters For Being Overpowered?

Why is an overpowered main character considered a bad thing?
Admittedly, I do like anime where the underdog comes forward and finally overcomes the seemingly unbeatable peril, but that doesn’t mean every main character has to be a wimp or a developing hero. It’s nice sometimes to have someone competent, in control, and at times even confident to follow along on their quest. In those instances, it isn’t tension that you are wanting to experience but rather the satisfaction of seeing someone overcome a challenge in a fairly capable manner. So, different emotional payoff but still entertaining, right?
So why do we criticise main characters for being overpowered?
First published June 2016.