So what do you suggest as an alternative to shooting at the middle of the biggest part of the bad guy that you can see?
Center mass is not taught in every class as the best target for no good reason, however if you study/train past the basics one learns it is not always your best choice. One example would be at close range say in your house. If the target/bad guy is the same size as me and I am aiming center mass the bullet will be traveling level to the ground and possibly going thru walls into a bedroom occupied by my kid, aim at the pelvic and I am shooting at a downward angle so the bullet hopefully will not go thru that same wall. Another example would be in a crowded area, maybe by dropping to the floor and shooting at an upward angle at the head you would have less of a chance to hit an innocent. lots of decisions to be made in a fraction of a second.