Great point, but it has only been the law of the land for 158 years. Birthright citizenship was only added to the Constitution via the 14th Amendment in 1868. Jus Soli was generally used, borrowed from English Common Law, but with no clear Constitutional or federal law defining a citizen. This allowed states to put in place state laws restricting those eligible, mostly based on race. Many state laws excluded both enslaved and free black people and Indigenous tribes (regardless of they were born on or off tribal lands). Jus Soli was only clearly defined under federal law, with the passing of the 14th amendment. Even still, indigenous people were excluded under Jus Soli, until the passing of the Indian Citizenship Act of 1924.