抱粗腿这个成语 originates from a folk tale in ancient China. According to the story, there was a fly that accidentally landed on a donkey's hind leg. Realizing its mistake, the fly quickly moved to the donkey's front leg. However, no matter where the fly landed, the donkey's legs were always thick and strong, "making" it difficult for the fly to find a comfortable spot.

Metaphorically, "抱粗腿" is used to describe a person who takes advantage of someone influential or powerful, hoping to gain benefits or protection. The reason why this expression uses "抱粗腿" (hug thick legs) instead of just "抱腿" (hug legs) is to emphasize the magnitude of the person's power or influence. Just like the fly searching for a secure place to land, people seek the support and favor of powerful individuals to enhance their own position.

This idiomatic expression vividly depicts the tendency of human nature to seek security and advantages by attaching themselves to influential figures. It carries a tone of opportunism and highlights the desire for personal gain. The donkey's "thick legs" symbolize the strong support and protection that influential individuals can offer.

In conclusion, the phrase "苍蝇落在驴胯上为什么是抱粗腿" is a metaphorical expression illustrating how people seek associations with influential figures for personal benefits. It vividly captures the idea of opportunism and the desire for security in human society.