It's in there somewhere, since VoiceOver uses it. I'd love it in my app as well, but it does not seem to have support for now.
They don't supply AVSpeechSynthesizer because no user wants an app speaking at them from their wrist, that is why Siri on the Watch is silent. It makes for a better experience, since people will most likely be interacting with your app on the go, and only be using it for a few seconds.
it is not that easy / black & white
AVSpeechSynthesizer should be available on the watch to allow interesting and relevant audio feedback after triggering an action - currently I don't see that this is possible
I found a way to play back audio files on the watch, which works fine - but then I need a MP3 or wav file.
and I think it is a waste of resources if having to generate voice audio files and transmit them through the network to the device, just because
AVSpeechSynthesizer is not available on the watch