The organism is a vessel to propagate the genes. Germ cells such as embryos usually have additional defenses and repair mechanisms, such as for DNA damage, that the somatic cells lack. Since the soma are merely there to help the replication of the germ line, they receive less investment.

No creature we know of has successfully evolved the capability to permanently stave off aging. Besides the inevitable issues with fighting entropy in a complex system, how would an ageless organism compete in the shifting landscape of an ecosystem as other creatures evolve around it? How would a non-aging organism adapt? It seems likely that continually creating new generations is the best strategy for genetic replication.

Now we are at the amusing point that genes may have unwittingly experienced their own skynet event. Modern science has given us the ability to not only gaze upon our creators, but manipulate them to do the bidding of ideas.