I'm assuming that the Prometheus story with the eagle is a way of explaining liver regeneration (please correct me if I'm wrong). But how did they know that the liver would grow back? I know they did surgeries in the ancient world but would they cut people back open and see if things grew back? Or is it not related at all? If it wasn't related then that's quite the coincidence.
Heres a formal academic article that really tackles the question.
Short answer: they very likely didn’t. They viewed the Liver as an organ that was connected to wisdom or the soul (or both). They likely viewed his liver being gored at everyday and yet regrowing as symbolism to his spirit getting tortured endlessly, and perhaps as irony as Zeus inflicting punishment to his Liver, thereby constraining or incapacitating his own wisdom and ingenuity, for retribution for giving fire (often interpreted as symbolism for wisdom) to mankind.