It's because birthright citizenship is actually very murky legally. The writers of the amendment were quoted and wrote extensively saying that the law was ONLY for former slaves and Indians. those two groups were essentially stateless in the 19th century. they were "subject to the jurisdiction" of the US but had no actual legal rights. It's like those poor fuckers born into cults who never get a SSN or birth certificate. they're completely fucked.
The case that established Birthright citizen ship has been expanded way past it's original scope to what we have today. Originally the ruling stated that if a child is born in the US to non-citizen residing legally in the US and conducting business, that child was a citizen.
If that original ruling was still in force the vast majority of spics in this nation would be non-citizens. unfortunately judges kept expanding it and expanding it until it covered all children.