The solution is simple: Let people pick their own hotword/phrase. Saying "okay Google" is incredibly awkward, and back when I wore Glass, I found "okay Glass" (where this format came from) equally awkward. Especially in public.
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
I believe the technical reason is that the wake word "ok google" is processed offline.
It's not only entirely possible to process any arbitrary phrase offline, it's entirely possible to process all speech on the device offline. That's just not conducive to Google's data collection-based business model.
I think it should be still possible technically to allow per installation wake words, just like local per install router passwords. Either during the first use or through some specific reconfigure process.