Because this keeps getting brought up again and again. Agree or disagree if you want.
When the constitution was written, natural born citizen had a different meaning than what Levin is professing. If you were born abroad and your father was not a citizen, and your mother was an underage American girl there were few back then who would have tried to argue that you were a natural born citizen. Those who say otherwise are subscribing to a revisionist version of history.
If you were born in the US or a US territory then yes. If you were born out side the US with one citizen parent it becomes more complicated.
The problem with Obama was that his US citizen mom lacked the residency requirement (18 years) to bestow citizenship to Obama at the time of his birth.
Nationality law is very completed because you have to refer back in time to what the law was at the time of birth. They never change things retroactively.
What brought this up again recently?