The most likely reason why an ancient Greek might worship more than one god is that

they believed that each god had specific powers and abilities, and worshipping multiple gods increased their chances of receiving help or favor in different aspects of their lives. Additionally, ancient Greeks believed that some gods were more powerful or influential in certain areas, such as Zeus being the king of the gods and ruler of the sky, while Poseidon was the god of the sea. Therefore, by worshipping multiple gods, they could ensure they had a diverse range of gods to call upon for different needs and circumstances.