Here's why:
* Saying verbs are verbs that describe the act of speaking. Examples include: say, tell, ask, answer, reply, whisper, shout, etc.
* "Moo" is an onomatopoeia – a word that imitates the sound it represents. It's the sound that a cow makes.
Therefore, "moo" is not a verb that describes the act of speaking, and it doesn't fit the definition of a saying verb.