Your liver is not protected by anything like the rib cage and your liver is a very sensitive organ.
So when you get kicked in the liver the liver is shocked by that liver kick or liver punch and then that leads to a lot of pain.
The kick to the liver or the punch to the liver shocks the liver which is the largest gland organ, and a center of blood circulation, and causes the victim to lose focus and drive, and can cause a breathless feeling in the victim.
Getting kicked in the liver or punched in the liver is supposedly more painful than getting kicked in the balls and just about on par with giving birth.
Although I'm a male so I cannot say how much painful it is to give birth.