Question

why will simple diffusion will not meet the requirement of human being

Answer 1

because human beings also need some molecules to move from place of lower concentration to higher for example sodium-potassium pump 
so this type of movement is known as active transport 

Answer 2

Reason why simple diffusion will not meet requirement of human being:

Human is a multicellular organism. In the multicellular organism the whole body is very complex and at the same time the demand of oxygen is very high.

So it is not possible to use simple diffusion process to circulate oxygen throughout the body as the simple diffusion is very slow process. If body uses simple diffusion to circulate oxygen in the body then it will take 3 years to reach oxygen to our organs which is not possible.