Because a birth certificate can be faked… or something:
Someone sent me a link to this article on the website of something called the Western Center for Journalism. I was hardly surprised, after I read the article, to find out that this organization was founded by Joseph Farah. They’re going beyond even birtherism and demanding Obama’s DNA:
This includes the determination of his actual identity, which requires genetic analysis. I started my investigation and analysis by deeming nearly every assertion as open to question, including the claimed identity of Mr. Obama’s parents. A certificate that a child was born to Stanley Ann Dunham and Barack Hussein Obama in Honolulu on 4 August 1961 might be true; but, assuming it’s true, it does not necessarily follow that Mr. Obama is that child. Whether he is or not requires genetic analysis.