Why? Because there are no living witnesses that can vouch for his authenticity, nor has he provided any official documents to prove it.
I was attempting to say why he has not provided any official documents -- and he won't until forced to do it.. by the courts? by the military?
His ideological parents are the 1960s Marxist-Alinsky hippie street rabble. They and their other ideological issue are the Rat Party (formerly the traditional, patriotic Democratic Party).
Their aim is to "Bring it all down, man."
They cannot be trusted without some kind of verification -- that requirement for Americans is new; and I remember every election since W.W.II.