After I wrote my above post, I reread your last posts and I think I understand your question a bit better. I think that your question may be based on the understanding of the term “metaphor” the way it is usually used. I think that usually when people speak of metaphors, they mean that they are using one word to mean something else (usually for poetic reasons). In that case, you have a valid question – why didn’t Chazal just use the word they mean?
However, that is now my understanding of how Midrashim work. That is the reason that I do not like to use the word “metaphor” as I stated above. The point is not that Chazal are using the term “tail” to mean something else. The point is that they are referring to the deeper, more conceptual meaning of the term “tail”. We live in a physical world and we are used to thinking in physical terms. But everything in the world really has a much deeper meaning.
When a child asks you a question about a topic that is difficult for a child to understand (for example, death or Neshamas or Olam Haba), you use terms and concepts that the child does not understand (such as death and Neshamas) and then you attempt to explain these terms and concepts on a level that the child can grasp to some extent.
Chazal are using the correct accurate word to describe Vashti’s malady (a tail). We then need the Mefarshim to try to bring it down to our level and explain what that means in a way that we can grasp to some extent.
The beauty of Midrash is that it gives us some idea of how much more there is to reality than what we ordinally perceive. It is our glimpse into Olam Haemes.
I hope that my explanation of Midrashim is accurate and that it helps to answer your question. Let me know what you think.