Because there is no self-knowledge.
The toad wants to eat swan meat, which means that a person has no self-awareness and wants to obtain something that is impossible to obtain. The saying originated from the Yuan Dynasty. The nobles of the Yuan Dynasty had always had the habit of eating swans. Moreover, among the nobility, swans were also a prized delicacy. This is not only a good thing that the nobles of the Yuan Dynasty seldom eat, but also a good product for sacrifice.
At that time, swans were difficult to hunt. Therefore, as farmers at the bottom, they rarely had the opportunity to eat swan meat. Even if they could catch a swan by chance, they might not know how to cook it. . Therefore, it gradually developed that when describing a person as being lowly and unworthy of possessing something noble, the person would be described as a toad wanting to eat swan meat.
Legend:
A long time ago, the Queen Mother held a peach party and invited all kinds of gods. The Toad Fairy was also invited. The Toad Fairy happened to meet the Goose Fairy in the Queen Mother's back garden, and was fascinated by her beauty, which moved everyone's heart. He was scolded by the Goose Fairy and sued to the Queen Mother.
The Queen Mother was furious and threw the moon essence basin offered by Chang'e in the Moon Palace at the Toad Fairy. The lower realm was punished as a toad, and the moon essence basin turned into a golden light and invaded the toad's body. The Queen Mother also regretted that she accidentally smashed the Moon Essence Basin and lost a treasure. She ordered Toad to return the treasure to Zhao after his ordeal was over, and then he could be re-instated in the Immortal Class and ordered the God of Thunder to supervise it.