Chattin' It Up

So every time I talk to my dad, he asks if Luke is talking yet. So Dad, this post is for you...

No, Luke doesn't talk. But here's what he does do:

-Pitiful cry # 1: he says "Hey, Boo-boo, boo-boo, boo-booooooooo. Heeeeeyyyy, Boo-boo, boo-booooooo." I have no idea where this came from, but McMoose and I both agree that it is hilarious and it has led to us calling Luke "Boo-boo."
-Pitiful cry # 2: "Oh, Mama, Ohhhh, Maaamaaa, Ohhhhhhh, Mmmmaaaammmaaaaaa." This one is more recent and he mostly does it when I've done something to piss him off--wipe his face, change his diaper, etc.
-When the cat purrs, Luke goes "Arrrrrr...arrrrrr...arrrrrr." He's been doing this since he was 4 months old, and technically, it was his first attempt at vocal communication. Figures it'd be aimed at the cat...
-When the dog barks, he goes "Oooohhh, ooooohhh, ooooooohhh," and then cracks up. What can I say, the kid has a weird sense of humor.
-He imitates the vacuum cleaner ("Hmmmmmmmmmrrrrrrrrr"), the leaf blowers that the maintenance guys have ("Harr, harr, harrrrrrrrrr"), and McMoose coughing ("Eh, eh, eh.")
-And earlier today, I was cleaning the oatmeal off his face, which always infuriates him and to make him smile, I started saying "Eek!" every time I wiped his mouth. Of course, he picked up on the game immediately, saying "Eeee! Eeee!" back to me in high-pitched voice and then grinning in oh-so-obvious pride.

And the rest of the time, he babbles and hisses and growls and basically narrates every moment of his life. So here's my question: how come he can mimic the dog, the cats, various loud machines and what he considers funny noises, but he can't mimic a simple "Mama" (except when he's crying, of course)?