Perhaps it’s due to the fact that she is the Evening Star— comes on bright and beautiful but then goes and disappears for the rest of the night. From what I gather, in the forms of antiquity we know about, it was not generally known that the Evening Star and the Morning Star were both Venus, thus making her look even less faithful. Every single night, she spends most of her time away! Fickle indeed. In contrast, the moon takes takes a whole month to go through her cycle, and only disappears entirely for a short time when she does.