The 13th, 14th, and 15th Amendments are why. You should read that Constitution you claim to have. Good day to you sir.The Citizenship Clause broadly defines citizenship, superseding the Supreme Court's decision in Dred Scott v. Sandford (1857), which had held that Americans descended from African slaves could not be U.S. citizens.
But The Supreme Court had an opinion in 1857 why isn't that the law today????