In 2012, Jinichi Kawakami emerged as the last surviving ninja grandmaster. Kawakami believed that the art of ninjutsu has no place in the modern age, as we have better weapons, the internet, and better medicines. Masaaki Hatsumi, another surviving ninja grandmaster, has not appointed an heir. Once Kawakami and Hatsumi pass away, ninjas will forever cease to exist.
Training starts years before birth in the egg half of the eventual person
This is the only logical explanation.