Because I've not seen any evidence indicating that he was not. A lot of wild speculation but no hard evidence at all.
Have you seen any evidence that he WAS? Like a valid birth certificate for example?
I can show you mine. It’s in a safety deposit box, but I can have it by tomorrow. How about you? How hard would it be for you to produce a copy of your birth certificate? Don’t you think it odd that Barry Soetero nee Barak Hussein Obama cannot produce his?