Actually based on typical life expectancies there should be a number of people who were present in whatever hospital Mr. Obama was born in. And given the infrequency of interracial births during the time in question, and the obvious fame of Mr. Obama today, it seems likely that someone who was present at the hospital would remember the event and be willing and perhaps interested in sharing their story.
The lack of such a witness only adds to the mystery.
Maybe he really WAS born at home in Hawaii with no independent witnesses as his maternal grandmother registered (I believe).