Because without a secret/alternative identity and persona, he would simply be a god-like alien among us. He would be the "[url=http://en.wikipedia.org/wiki/Justice_Lords]Justice Lords[/url]" version of Superman. One where he deemed that his superiority and might made him "right". To not have/be Clark Kent would mean that he deliberately and consciously distanced and separated himself from his adopted home and its people. Based on my understanding of the character and his statements over the decades, Clark Kent is not Kal-El's "front" or a hollow put-on character, Clark Kent is a part of who Superman is. Kent is his humanity. His acknowledgement and connection to the human race. Clark Kent (as raised by the Kents) is the characters "nurture" and his Kryptonian abilities is his "nature". As an adopted child myself, the difference and the importance/value of each was crystal clear to me from the start.