One answer I've read in The Spark In The Machine[1], is that our cells' organizational energies (or Qi in Chinese Medicine) is not strong enough as we grow older.

Only when someone is super young (when their qi is super strong) can they regrow body parts and very small ones (the tips of the fingers!)[2]

""" Doctors have seen the effect in humans without quite understanding how it happens. "Kids will actually regrow a pretty good fingertip, after amputation, if you just leave it alone," says Dr. Christopher Allan, from the University of Washington Medicine Hand Center, who wasn't involved in the research. """

EDIT: yeah i know its HN so i'll probably get downvotes, but as far as modern science is concerned we still know very little about embryology and what actually controls the beautiful orchestration of a single cell developing into a full human in 9 months. The idea of an organization energy that directs embryological cellular function is now starting to be investigated in western science [3]

[1]https://www.amazon.com/Spark-Machine-Acupuncture-Explains-My...

[2] http://www.npr.org/sections/health-shots/2013/06/10/19038548...

[3] https://vimeo.com/26761300