Why does an iPad's qwerty keyboard lack a ctrl and an alt key?

Economy and alternative gestures is the answer I guess.

The size of a touchscreen like an iPad or iPhone means that interfaces need to be designed with economy in mind. This means no extra buttons if there's a viable alternative.

In addition, the fact that you have alternative gestures, on-screen buttons and navigation means features can be accessed without the constraints of a keyboard.

In fact, some gestures on a touchscreen are much more intuitive than using a keyboard command. For example, selecting text to copy and paste is done via a long touch on a highlighted text area, rather than a combination of keys...