It's too much work. Time equals money, and this will take a lot of time. Our brains can't handle a voice-enabled Web. Phone numbers are only seven digits long because that is the maximum amount of information we can store in our short-term memory. Yahoo just dictated that you want to follow.
Nobody cares anyway.
People will quickly realize it's easier to read e-mail on their wireless devices than listen to a computer drone over the phone. The Prediction Someone will eventually work out the kinks and offer a quality speech recognition product that has realistic applications, but there's no need to jump on the bandwagon until you're sure the wheels won't come off.