Here's why:
* Simile: A simile uses the words "like" or "as" to compare two things.
* Idiom: An idiom is a phrase that has a figurative meaning that is different from the literal meaning of the words.
"Fit as a fiddle" means to be in excellent health and physical condition. It doesn't literally compare someone to a fiddle. It's a phrase that has evolved to express that idea.