Question

Why do only humans have a very complex digestive system

Answer 1

because the food which we intake  is of complex type and to digest such we need a powerful and complex digestive system every food type is important for us ..
it is difficult to digest fat but our complex digestive system has solution to it
so that is why we have such..

hope it helps 
thankyou

Answer 2

The digestive system of humans is composed of various organs and glands.The human body is very complex and so is the digestive system.There is no reason for the complexity.It is an adaptation to maximise the nutrient absorption in the body so that the body can utilise maximum of their food for energy production.