That's on purpose. Most genealogical studies show that humans in oral societies are bad at reckoning beyond four generations back. Moreover, things like the Trojan War and the Argonaut (as well as the Theban Wars and the Calydonian Boar Hunt) tended to attract heroes by their very nature: wars and hunts and adventure bring allies, so many heroes were grafted onto these stories when originally they would have been independent.
