Here's why:
* Onomatopoeia is the formation of a word from a sound associated with what it refers to. Examples include "buzz," "meow," "crash," and "sizzle." These words mimic the actual sounds they represent.
* "Smack" is a word that describes a specific type of impact, but it doesn't directly imitate the sound. While it's possible to imagine a "smack" sound, the word itself doesn't inherently evoke the sound.
Examples of onomatopoeia related to impact:
* Bang
* Wham
* Thud
* Crash
These words are more directly imitative of sounds than "smack."