He's being sarcastic. If you are going to talk to your phone, why not just eliminate all the extra steps and actually make a phone call?
Possible reasons:
1) You are on prepay/pay-as-you-go and don't want to spend money on a call
2) It's a simple yes/no question that doesn't warrant a phone call
3) The other person is in a noisy environment and cant hear you on a call
4) You just don't like calling
Also, speech to text would offer the ability to do so much more than just write messages, as I mentioned above.
EDIT: I'm not trying to be argumentative, these are genuine reasons. Also, I completely missed on the sarcasm