"Nobody really knows the answer," says David M. Gardiner, a professor of developmental and cell biology at the University of California-Irvine, who is a principal investigator in the UCI Limb Regeneration research program. "Regeneration is a fundamental, basic, biological property, just like reproduction."
As Gardiner explains, humans actually do have regenerative abilities. Our bodies continually rebuild themselves at the cellular level, and have an impressive capability to fix damage and heal wounds. We can't grow back a lost limb, but as a 2013 article in Nature documented, children sometimes are able to grow back fingertips that have been accidentally amputated. And an adult human can regenerate a portion of his or her liver, if that organ is damaged.
"If we didn't have the ability to repair ourselves, we couldn't survive," Gardiner notes. "But if we can regenerate in pieces, then why can't we make organs?"
What's frustrating is that we all had that ability when we were in the womb. Humans are built, piece by piece, by embryonic stem cells, which are highly pluripotent -- that is, able to divide and differentiate into various other sorts of cells, from nerve cells to muscle cells to blood cells.