Because the Court has only discussed the issue in dicta. Natural-born has never been part of a Supreme Court decision's central holding therefor precedent (and stare decisis) isn't established. Dicta can be used for lower courts and the Supreme Court as persuasive precedent, but not binding precedent. So, they can weight it, but they're not bound by it.
If you want a better, more detailed explanation, an Illinois law professor, and one of the foremost authorities on semantic originalism, Lawrence B. Solum, wrote a piece that appeared in the Michigan Law Review several years ago. It can be found searching google with those parameters.
Now, for someone to figure out how to get standing in any court.