Well, for the purposes of the games, that's how all Kangaskhan are. However in the anime, baby Kangaskhan look different.
^ A baby Kangaskhan.
Since Kangaskhan don't evolve, it'd be strange to make newly born ones look like that in the game. When will they turn into the 'bigger', 'grown up' Kangaskhan? There's no pre-programmed level for them to do so, and even if they did, wouldn't that be an evolution, even though Kangaskhan aren't supposed evolve?
Its probably because Kangaskhan was designed for the anime, and then when time came to input the species into the games, they only gave it the 'grown up' sprite, so that's why it already has a baby.
It would actually make more sense that this baby Kangaskhan was a Baby Pokemon, but since that concept wasn't introduced until Gen II, Kangaskhan became a single evolution Pokemon, and remained as such because suddenly renaming a Pokemon's previous 'baby' form would probably seem strange...