why caterpillars need to shed their skin when they grow bigger but we humans do not?

Caterpillars are larval forms of insects such as butterfly. Caterpillars shed their skin 3-5 times while they grow and this process is called molting.  Their skin never grows as skin of human does, so while growing they have to crawl out of their skin when their skin becomes too tight. After first molting they bring new skin cover and in the mean time some caterpillars also eat their shed old skin. After  they grow more, then again shed their skin. 

 In case of humans, shedding of whole skin does not occur as when human body grows, skin cells also grow. Shedding of only dead skin cells occur in case of human beings.