The answer to this is that, “because it is painful”. If you have talked to any mother who had a natural delivery, they will explain their pain as a 100 times intensified version of their menstrual cramp combined with a feeling of being stabbed, severe pressure and for some intense waves that feel like diarrheal cramps. This pain is caused by contractions of the muscles of the uterus.
Yet there will be a few that say that it was pain-free and that they didn’t know that they were in labor. The amount of pain that one experiences during childbirth depends on factors like the size of the baby, the speed of the labor, the strength of one's contraction, whether the baby is face up or face down and so on. With this pain women generally experience pressure on one’s back, bladder and bowels. This can also vary from person to person and also with the number of pregnancies for each person.
People say that pregnancy is painful because it is painful. Just imagine the amount of pain that the mother will be in when the stomach starts to contract. It can also happen anytime so the mother might not be too prepared for it.
There is a pressure that is felt on the bladder of the mother as well as the bowels. There are some mothers who are lucky if they are able to release their child immediately.
There are some mothers who would go in labor for hours and they would already be too tired of pushing. Some mothers who give birth once say that they do not want to give birth again.