Here's why:
* Metaphors and similes are figures of speech. They involve comparing two things that are not literally alike. Metaphors do this directly, while similes use words like "like" or "as."
* "a b" is a mathematical notation. It represents multiplication, meaning "a times b." This notation has a specific, literal meaning in mathematics.
Think of it this way: "a b" is like a shorthand for the operation of multiplying the values of "a" and "b." It doesn't involve any figurative language or comparison.