The Nintendo DS is a small portable, one meant to be carried around in your purse or pocket. It doesn't have a keyboard because it doesn't need one. Unless you get Pokemon Typing, that is.
This new game that's designed to teach you how to type comes with a wireless Bluetooth keyboard that can pair up with the DS. Sure, you could learn to type on a normal-sized keyboard attached to, you know, a computer. And this makes the DS far less portable. But hey, maybe you want to learn how to type on an unnaturally small keyboard? If so, this is your game.