This seems the most concise and precise answer to the question of "why". At some stage, somehow, self replicating molecules appeared in the primordial soup*. Ev

This seems the most concise and precise answer to the question of "why". At some stage, somehow, self replicating molecules appeared in the primordial soup*. Everything since then has just been improvements (or maybe complete overhauls) to that process. *How (or why) this step occurred is another intriguing topic.