Because she wasn’t his mother. As to why the Dunhams took him in, who knows?
There is only one explanation.
Barack Hussein Obama II is ... “Rosemary’s Baby”!
http://www.imdb.com/title/tt0063522/
I’m not convinced that she wasn’t, but I’m not convinced that she was either.
Obama resembles her a lot and looks a lot like her father as well, so, I’m not sure what to make of the whole thing.