I used to think the same thing. My SIL's 3 kids are all older than mine. They would refer to the girls (both of them) as "Sissy." They referred to all animals (stuffed animals, toys, farm animals, etc) by the noise they make. "See those Moo Moo's over there?" "Do you want your Quack Quack?" Seriously! How hard is it to call a child by her name (why is every girl called "Sissy?") and teach kids the actual name of the animal - cows, ducks, pigs, dogs. It's not like these words are difficult to say.
I guess it's a cultural or regional thing. I thought it was crazy (I still think it