The lack of any evidence to the contrary.
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There is no credible public evidence for where Barack Hussein Obama, a/k/a Barry Soetro [1], a/k/a Soebarkah[2] was born.
It isn't wise to put your trust in and believing a reprobate community organizer and the corrupt HI govt who has issued fraudulent birth certificates in the past.
One of the more famous examples is their giving a territory of HI birth certificate to Sun Yat Sen, the leader of the Chinease Nationalist Party who was born in Xiangshan China [1][2][3].
...based on his word:
The reason Sun Yat Sen needed a HI BC is summed up here. And, interestingly, Dr. Sen had also obtained a Japanese passport a number of years after.
Of course, he has another problem. Assuming BHO SR. is in fact his legitimate, legal birth father, he inherited his foreign father's foreign subjectship by birthright and therefore owed allegiance to a foreign government...yet another issue with his Article II Constitutional eligibility [1][2][3[4][5].