The wakewords are chosen so that they can be trained to work out of the box with a variety of cultures and accents. To re-train a new word would mean you would

The wakewords are chosen so that they can be trained to work out of the box with a variety of cultures and accents. To re-train a new word would mean you would have to train it out of the box which could be a pain to their vision of a unified and simple system