I am sure that many of you know people who have lost control of bowel movements. This miserable feeling would be how every human would feel for the first years of its life. Certain Animals are born fully functional. The answer is, probably, that G-d created man as a social creature. If a child was born independent he would never become social.
Social life is very important to people.
The incredibly long development of a person serves him best to learn all that is expected of him by society. He can then grow up and do good with his life. Another point which we can see the wisdom of G-d is in our bodies. It is incredible that such a complicated system of body organs works for so well for so many years. For all the years that a person lives his lungs collect up Oxygen which is added to the blood which is pumped by the heart and cleaned by the liver.