Few things in ancient Israel or current day Australia could evoke a similar level of repulsion as the thought of cannibalism. The practice is outlawed today in our culture, just as it was in ancient Israel.
This begs the question, why would Jesus use an idea that seems so much like cannibalism to express something about his person… his identity?
Jesus wanted to make it clear that he was not simply like every other preacher/teacher that has arisen in the history of Israel. He was not simply one of the prophets. There is something entirely unique about who he is and how we are to relate to him.
The use of this idea means his audience (then and now) are not able to simply smile and nod and comment politely.
Jesus requires a more definitive response.