Because he actually cared about the sanctity of the Office of the Presidency and his own perceived legitimacy?
I agree that if Obama were an honorable man, he'd take the trouble to get a long-form BC, even though it is a hassle and not necessary to prove his eligibility.
Unfortunately, being an honorable man who cares about the sanctity of the office is not an eligibility requirement, and Obama is most decidedly not an honorable man.
Of course, since you don't trust the word of government bureaucrats, I seriously doubt you or any of your ilk would be satisfied even if he did the above.