Well, that's because Google isn't going to respond to "hey Siri..."
That being said, you can do this on Android phones with Google Now, and have been able to do it before the iPhone enabled this feature. I was under the impression that Apple enabled it as a response to Google Now having done this for a while.
You have to enable your phone to do this though. It is not setup by default. You have to go into Google Now and go through the settings to enable Google Now to unlock your phone. I have been unlocking my phone with "Ok Google" for a very long time.